Hiring a Retell AI Developer? A Letter Before You Sign

A letter to the owner hiring a Retell AI developer: book or take requests, whose account it runs on, where the date comes from, testing and cost per minute.

By Carlos Familia, CRM & AI automation expert. Updated October 17, 2026. 5 min.

If you run an appointment business and a developer has offered to build your phone receptionist on Retell, this letter is for you. I'll start with one I built for a med spa in California, because a single decision on that job shaped everything after it.

That spa books in its own software, separate from the phone system. So the receptionist doesn't book anything. It captures the request, gives the caller a fixed spoken reply for the reason they called, and leaves the slot for staff to confirm. Everything I tested afterward followed from that choice.

Here is what to look for. A good Retell developer asks whether the agent should book or take requests before they quote, keeps the account in your name, shows you the tests that failed, and can price a minute of your calls. The rest of this letter is how you check each one.

Decide first: will it book, or take requests?

Booking into your calendar means the agent writes to a system you depend on. A wrong write is a wrong appointment. Capturing a request is smaller: the caller still gets an answer, and a person confirms.

Neither is better in general. Pick based on how well your booking software opens to outside tools. If a developer quotes before asking, they have made the choice for you.

Open the Retell account yourself

Whoever holds the account pays the usage and sees the calls. Retell's pricing page lists pay-as-you-go voice agents at $0.07 to $0.31 per minute, phone numbers at $2 a month and $10 in free credit when you sign up.

Create the account with your business name and your card, and have the developer work inside it as a user. Then the balance, the call history and the bill are yours to see. Ask where it can call, too. On a Retell build for a lead-gen agency's demo line, the platform could not dial Dominican Republic numbers at all, which matters if your customers live abroad.

Ask how your agent will know today's date

A voice agent doesn't know the date unless someone tells it. On the med spa build, a small webhook fires when a call connects and passes in the current date plus the upcoming open days, so it has what it needs to answer "are you open Thursday?"

Relative days need the same care. On a facial spa's phone agent, the server turns "tomorrow" into a real date before the booking tool sees it, so the model never does date math. Ask your developer where the date comes from, and ask to hear it work on a call placed late in the evening.

Expect fixed wording where it must be exact

Not every sentence should be improvised. For each reason a caller might give, I wrote one fixed spoken reply, so the wording is the same on every call. A developer who says "the AI will figure it out" for everything is describing a risk, not a feature.

Ask to hear the calls that failed

I ran a 24-question suite against that receptionist two ways: as text chats, and as phone calls placed by a second AI agent that graded every answer. The phone core set scored 14 of 15 on a 467-second call. By text, the core set landed at 13 or 14 of 15, and a second, extra suite scored 8 of 9.

The misses were the useful part. One run filed a first-time caller as someone already on the books. Another lost a confirmed request when the caller raised an emergency in the same breath. Each graded run costs me roughly $1.80 by phone and $0.23 by text, so budget is not a reason to skip it.

Ask your developer for their failing tests, and ask how they keep tests away from your live systems. Retell's testing documentation says an unmocked custom function, code tool, calendar tool or MCP tool calls its real production endpoint during a test, while transfers and SMS are always faked in a simulation. So a calendar tool needs a mock or a test calendar, and a transfer needs a real phone.

Ask which words were boosted

Speech recognition can be told to favor certain words, like your treatment names. That helps until it doesn't. On the med spa agent, one boosted keyword pulled an email domain toward the wrong spelling. The agent spells addresses back to the caller, and that is where the error showed.

Ask which words were boosted, and whether emails and phone numbers are read back to the caller.

If your callers speak Spanish, have a native speaker listen

On the spa's agent, the test caller who spoke Spanish first got replies in English, and that took a fix. For a clinic project in the Dominican Republic, the Spanish voice "pronounced everything wrong" because an American-accent voice was running on a fast English model, and the model drifted back to English until the prompt carried a strict Spanish-only rule.

Ask for a recording of a full call in Spanish, and have someone you trust listen to the whole thing.

Ask what happens to call number 21

Retell's pricing page says every account includes 20 concurrent calls, with extra capacity at $8 per concurrent call per month. Ask how many calls could ring at once on your busiest morning, such as the Monday after a long weekend, and what the 21st caller hears. A fallback to a message line beats a busy signal.

Budget the minutes, not only the build

The developer's fee is one line. Retell's page also says billing covers the entire call, silence included, because speech-to-text stays active, and its FAQ answers yes to being charged during hold time. Your bill follows your call length. My arithmetic for a few volumes is in AI receptionist cost.

Take these six questions to the conversation:

1. Does the agent book, or take requests that staff confirm?
2. Whose name is on the Retell account, the number and the card?
3. Where does the agent get today's date, and who resolves "tomorrow"?
4. Can I see the failed tests, and was the transfer tried on a real phone?
5. Which words were boosted, and are emails and numbers read back?
6. Have I heard a full call in my callers' language?

For the general interview, use the AI voice agent developer questions. For clinics, read AI voice agent in healthcare on what a receptionist should never handle, and the AI voice agent red flags on vendor risk. If you want the scope in writing before anyone starts, my AI automation scope of work template covers it, and the rest of a spa's system is in med spa CRM.

P.S. I work from the Dominican Republic, close to US Eastern time, and I build in English and Spanish. If you already have a proposal from another Retell developer, bring it to a call on the calendar below and we can read it together.

About the author

Carlos Familia, CRM & AI automation expert. GoHighLevel, n8n, AI agents, servers. Full-stack developer.

I build CRM systems in GoHighLevel, n8n automations on my own servers and AI voice and chat agents for service businesses. Full-stack developer: I write the workflow, the API integration, the database and the server it runs on, and I test every build with real journeys before handoff.