The problem that sends people to Retell
It usually starts when one prompt stops being enough. A new caller should be qualified, a booked customer wants to move an appointment, someone else has a billing question, and a single block of instructions keeps blending the three. Retell is known for flow-style agents, so it is the first name people search.
HighLevel covers the same need inside the sub-account. Its help center describes the Flow Based Builder as a visual flow canvas for Voice AI, built from connected nodes, and the finished agent deploys to an eligible phone number or a Number Pool in that sub-account.
Our own default follows from that. We build voice agents on HighLevel's native Voice AI unless there is a reason the job can't stay there, and needing branches is not one of those reasons.
What the Flow Based Builder gives you
A new flow opens with two nodes already joined: Start Call, then an AI Agent. Everything else is added from there.
- AI Agent nodes. Each one has its own instructions, its own model and its own tools, so the greeting node and the booking node can be written and tuned separately.
- Router nodes. A router looks at the conversation and chooses which path the call takes next.
- Custom API nodes. These connect the live call to a system outside HighLevel.
- Built-in options on the canvas labelled Search Knowledge Base, Search the Web and MCP Server.
- MCP is optional. HighLevel's overview says a flow made from native nodes and actions doesn't need it.
The naming trap: Agent Studio vs the Voice AI canvas
Older notes and some tutorials describe these parts as LLM and API Tool. Those are Agent Studio names. HighLevel's Voice AI overview, updated in October 2026, calls them AI Agent and Custom API.
The names matter less than the assumption behind them. Agent Studio has its own API, and nothing we have checked shows that it builds the same graph as the Voice AI flow canvas. Build voice flows on the Voice AI canvas, use the labels your screen shows, and expect them to move: HighLevel has changed this before, so check the current screen before you copy any node list into a build plan.
If routing is the only thing pulling you toward Retell, HighLevel already does it.
Building a flow agent in HighLevel
These are the steps from HighLevel's own setup article, in the order we follow them.
- Go to AI Agents, then Voice AI, then Agent List, and click + Create Agent.
- Choose Flow Based Builder, continue, and save the starter flow with its Start Call and AI Agent nodes.
- Write the prompt for each AI Agent node and give it only the tools that node needs.
- Add a Router wherever the call splits, and connect every path through to a termination node so no branch is left open.
- Set up any integration a Custom API node calls, including its login or key on the other side.
- Test each route on its own, then assign an eligible number or Number Pool so the agent can take live calls.
Rather have it set up for you?
We'd build your voice flow inside HighLevel, every node and route written out for your approval first, starting with a 30-minute 1:1 with Rob.
Where Retell fits
A common belief is that Retell always means opening a Twilio account. Retell's own documentation says otherwise: it sells managed phone numbers that need no separate phone provider account, and it can also connect to a provider you already use.
That still makes it a second product, with its own numbers or its own phone connection, sitting beside the HighLevel account where your contacts and calendars are. We haven't compared the two feature by feature or on cost, so this guide won't rank them. The narrow answer stands: a flow agent does not require Retell.
Before the flow takes live calls
A flow can look finished on the canvas and still fail on the first real call. These are the checks we run before a number is pointed at it.
- Every Router branch tested with a caller who should land there, not just the main path.
- Every path ending at a termination node, so a call never stalls between nodes.
- Each Custom API integration authenticated in the account the agent runs in.
- An eligible number or Number Pool assigned. Without one the flow can be built and tested, but it won't answer.
- Node names and options compared with your own screen. The research behind this page did not inspect a live account.