Retell vs Vapi vs Bland vs Synthflow: AI Calling Platforms Compared (2026)

Updated October 22, 2026. Pricing, docs and reviews checked on each vendor’s own website and on public review sites on that date.

The short answer

Retell, Vapi, Bland and Synthflow are platforms for building your own AI phone agent. They give you the voice and language models and the phone connection; you write the prompts, set up numbers and caller ID, connect your CRM and handle calling rules yourself. The right one depends on how much your team wants to build:

• Vapi: the most developer-focused. $0.05 a minute for hosting, with speech, model, voice and carrier minutes billed on top (about $0.10 to $0.14 a minute in practice).

• Retell AI: self-serve and pay as you go, $0.07 to $0.31 a minute, with paid add-ons for outbound such as branded caller ID ($0.10 per call).

• Bland: enterprise-focused. $0.12 to $0.14 a minute on self-serve plans with daily call caps; calendar booking and number pools are Enterprise only.

• Synthflow: enterprise contracts only, starting at $30,000 a year.

• Outbound System: a ready-to-run AI calling platform for outbound, at a flat monthly price ($1,499 to $3,999/mo for 250 to 1,000 dials a day). Local number pools, do-not-call checks, voicemail handling, calendar booking and CRM sync are built in, so there’s nothing to build. Disclosure: this is us.

If you have engineers and want full control of an inbound or outbound voice agent, look at the first four. If you want outbound calls booking meetings without building a voice stack, look at the last one.

Side-by-side comparison

Price and who it’s for

Numbers, spam and compliance

Calling hours, booking and speed

Reply speeds are each vendor’s own claim, not an independent test. Synthflow’s figure describes its phone network, not how fast the AI answers.

What it really costs per 1,000 minutes

Per-minute prices look small until you add the pieces. Using each vendor’s own price list:

• Vapi: $50 of hosting, plus speech, model and voice at the providers Vapi shows, comes to about $82 to $129 before carrier minutes (Vapi’s own calculator). Premium voices and models push it higher. Pro support adds a $999/mo minimum.

• Retell AI: about $110 at its default settings ($0.11/min). Outbound adds $5 per 1,000 dials for batch calling and $0.10 per call for branded caller ID. On 30-second calls, that caller ID fee alone works out to about $0.20 a minute.

• Bland: $140 on Start or $120 plus $299/mo on Build, with transfer minutes ($0.04 to $0.05) on top. Its pricing page says telephony is billed separately; its homepage says one rate covers telephony.

• Synthflow: no per-minute price. The $30,000 a year minimum equals $2,500 a month before usage is scoped.

• Outbound System: one flat monthly price based on daily dials. Numbers, voices, dialing, compliance checks and CRM sync are included.

What you have to set up yourself for outbound calling

Outbound is harder than inbound. Before a platform agent can call a list, someone has to handle:

1. Phone numbers and caller ID. Free Vapi numbers can’t dial out; Vapi’s docs say carrier caller ID verification “typically takes 5-7 business days.” Retell’s verified-number application takes “1-2 weeks.”

2. Spam labels. Calls from one number at high volume get labeled “Spam Likely.” None of the four documents a self-serve number pool with rotation; Bland offers number cycling on Enterprise.

3. Do-not-call and consent rules. Vapi’s docs: “You are responsible for lawful use.” Retell’s do-not-call handling is “off by default,” and its docs say “Batch calls and inbound calls don’t check” it.

4. Calling hours across time zones. Retell batches run on one time zone (default Pacific). Synthflow windows use the agent’s time zone. Leads in other states get called too early or too late unless you split campaigns.

5. Voicemail and phone menus. All four detect voicemail in some form; you still configure what happens next.

6. Booking and CRM. Calendar booking and CRM logging are built in on Retell and Synthflow, Enterprise-only on Bland, and built through tools and APIs on Vapi.

7. The script. On every platform, you write and test the prompt.

Vapi

What it is: a developer platform for building voice agents. Its homepage headline: “Voice agents for builders.”

Pricing: $0.05/min hosting plus provider costs “with no markup,” $5 in free credits, 4 concurrent calls on the free tier (its docs say 10). Pro support starts at a $999/mo minimum.

Outbound setup: numbers imported from a carrier, caller ID verification through the carrier, campaigns of up to 10,000 contacts uploaded by CSV, and compliance on you.

What users complain about: lag (“a 3-5 second pause after each turn,” Trustpilot, March 2026), a builder that loses settings, slow support on entry tiers, and costs stacked on top of the advertised $0.05. Trustpilot score: about 2.4 from 15 reviews.

Who it fits: engineering teams that want to assemble and control every piece of the stack.

Full breakdown: Vapi vs Outbound System.

Retell AI

What it is: a self-serve platform to “Build, update, test, and deploy AI voice agents,” with APIs, SDKs and templates.

Pricing: $0.07 to $0.31/min pay as you go, $10 in free credits, 20 concurrent calls included ($8/mo for each extra). Outbound add-ons: $0.005 per batch dial, $0.10 per branded call, $2/mo per number.

Outbound setup: batch calls with one time zone per batch, a verified-number application to fight “Spam Likely” labels, do-not-call handling you switch on yourself.

What users complain about: outages and lag on long calls (a customer spending “$10k a month” on its forum, September 2026), support after payment, and batch-call problems, including calls placed at 2:32 AM outside the set window. Trustpilot: 4.9 from about 810 reviews; G2: 4.8 from 2,638.

Who it fits: technical teams that want pay-as-you-go pricing and are comfortable building and maintaining agents.

Full breakdown: Retell AI vs Outbound System.

Bland

What it is: an enterprise voice AI platform running on its own models, positioned for regulated industries (“Voice AI for the calls that can’t go wrong”).

Pricing: Start at $0.14/min (100 calls a day, 10 at once), Build at $0.12/min plus $299/mo (2,000 calls a day), Enterprise custom. Calendar booking, live and warm transfers, number pools and guardrails are listed as Enterprise only.

Outbound setup: calls go out from a shared Bland pool by default; local caller ID is a paid add-on; number cycling goes through your Bland rep. Bland describes a 28-day deployment framework for Enterprise.

What users complain about: a robotic voice and slow reaction to interruptions, unresponsive support outside Enterprise, and agents or knowledge bases wiped by platform changes. Independent reviews are thin (2 on Trustpilot).

Who it fits: large companies that want an enterprise contract, compliance certifications and engineers from the vendor.

Full breakdown: Bland vs Outbound System.

Synthflow

What it is: an “enterprise-ready” voice AI platform with its own telephony and forward-deployed engineers.

Pricing: “Enterprise contracts start at $30,000 annually,” scoped with sales, with implementation included. Self-serve plans are closed to new customers.

Outbound setup: campaigns are created through the Platform API, calling windows run on the agent’s time zone, and caller ID must be a purchased or verified number. No number rotation or do-not-call feature is documented.

What users complain about: agents going off script (“severe hallucinations,” Trustpilot, May 2026), billing after cancellation, features locked behind enterprise pricing, and slow support. Trustpilot: 4.1 from 215; G2: 4.5 from about 1,000.

Who it fits: enterprises that want a vendor-led implementation under an annual contract.

Full breakdown: Synthflow vs Outbound System.

Outbound System

What it is: a ready-to-run outbound AI calling platform, built in-house. Disclosure: this is us.

Pricing: $1,499, $2,499 or $3,999 a month for 250, 500 or 1,000 dials a day, month to month. No per-minute fees.

How it handles outbound:

• Speed and voice: the AI replies in under 300 milliseconds, lets people interrupt, and says numbers and acronyms the way a person would.

• Spam flags: every client gets its own pool of 50 to 200 local numbers, each capped at 10 calls a day, with caller ID names registered. Our numbers don’t show up as spam.

• Compliance: every number is rechecked against the national Do Not Call list and a known-litigator list right before each dial, and calls run in each lead’s local calling hours.

• Voicemail and phone menus: the AI listens live, hangs up on voicemail instead of leaving robotic messages, and recognizes phone menus and call screeners.

• Scripts: every script is checked before launch, and claims your business can’t back up are flagged.

• Booking and CRM: consent before booking, email read back, meetings in Calendly, Cal.com, HubSpot or Monday, and new leads in from Zapier, Make, n8n or GoHighLevel.

• Lists: upload your own, or we build a B2B list with mobile and direct numbers.

Know before you buy: outbound only (it doesn’t answer inbound calls), no live transfers to your team, no text messages, and it’s not the cheapest option if you only make a few hundred minutes of calls a month.

Who it fits: businesses that want outbound AI calls booking meetings without building or maintaining a voice stack. You can hear it first: enter your number on our AI calling page and the AI calls you.

How to choose

• You have engineers and want full control: Vapi or Retell.

• You need enterprise certifications and a vendor team: Bland or Synthflow.

• You want outbound calls booking meetings this month, without building anything: Outbound System.

• You’re comparing on price: model your real minutes, not the headline rate. Add numbers, caller ID fees, concurrency and support plans before you compare.

FAQ

Is Vapi cheaper than Retell?

On paper, Vapi’s $0.05 a minute is lower, but it covers hosting only. With speech, model, voice and carrier minutes, Vapi typically lands around $0.10 to $0.14 a minute, close to Retell’s default of about $0.11. Outbound add-ons (Retell’s branded caller ID, Vapi’s support plans) change the math more than the base rate.

Which AI calling platform has the lowest latency?

Each vendor publishes its own number: Bland says about 200 milliseconds, Vapi “sub-500ms,” Retell “~600ms,” and Outbound System under 300 milliseconds. These are self-reported. The best test is to call each one yourself and listen for pauses and how it handles interruptions.

Which is best for outbound sales calls?

The platforms can all place outbound calls, but outbound adds work: numbers that don’t get flagged as spam, do-not-call checks, calling hours in each lead’s time zone and calendar booking. If you don’t want to build those, a ready-to-run outbound platform like Outbound System covers them at a flat price.

Do I need a developer to use Vapi, Retell, Bland or Synthflow?

For Vapi, realistically yes. Retell and Bland have no-code builders, but outbound campaigns, CRM connections and compliance still take technical setup. Synthflow includes implementation in its enterprise contract.

What happened to Air AI?

Air AI is no longer selling an AI calling product. The FTC announced a settlement in March 2026 that bans the company and its owners from marketing business opportunities, and the air.ai domain now shows a different product.

How do AI callers avoid being marked as spam?

By spreading calls across many local numbers, keeping daily volume per number low and registering caller ID names. Outbound System does this for every client with a pool of 50 to 200 local numbers capped at 10 calls each a day.

Bottom line

Vapi, Retell, Bland and Synthflow are building blocks: powerful if you have the team to assemble and maintain them, expensive in time if you don’t. If what you want is outbound AI calls that sound human, don’t show up as spam and put meetings on your calendar, without building anything, talk to us or hear the AI call you first on our AI calling page.