For AI phone calls, AgentLine, Twilio and Vapi offer different integration boundaries. AgentLine provides a hosted phone agent and number through REST, MCP or a skill. Twilio offers telephony building blocks including managed speech through ConversationRelay. Vapi provides configurable voice-agent APIs. Choose according to what your application needs to control.
This is an AgentLine-authored guide reviewed on October 18, 2026. It does not claim that one platform always costs less or takes fewer hours to integrate.
Twilio has more than one implementation path
Twilio ConversationRelay handles speech recognition and synthesis while your application exchanges messages over a WebSocket connection. That differs from building an audio pipeline around Media Streams. Do not estimate every Twilio integration as a from-scratch speech stack: scope the chosen Twilio product before comparing engineering effort.
Consider this route when you want to own application behavior around the call and it fits your existing telephony architecture. Budget for the application service, monitoring and failure handling as well as usage. Test disconnections and delayed application responses, not just a successful greeting.
AgentLine's hosted call boundary
An AgentLine application creates an agent, provisions a number and starts a call using agent_id and to_number. Hosted voice follows the approved system prompt. Your runtime joins for explicit tasks or lookups through the supported integration; it does not need to generate each spoken reply. The REST walkthrough includes the full sequence and transcript retrieval.
This fits an agent that needs phone access without owning the ordinary voice loop. If a call must access your CRM or calendar, your integration still needs to run reliably. Hosting the conversation does not make external business systems available automatically.
Where Vapi fits
Vapi's call API and voice-provider configuration give developers another managed route. Evaluate it when choosing voice providers or configuring assistant behavior is part of your requirements. Compare request resources, event payloads and tool execution before migrating a workflow. The voice selection guide contrasts the configuration models.
Compare the same workload
- Define the required tasks, number coverage, call direction and human escalation behavior.
- Use the same test script on your own phones and keep prompt versions with the call IDs.
- Test an interrupted reply, slow business lookup, unavailable destination and failed external write.
- Compare the resulting transcript with the downstream system's actual state.
- Add number rental, speech/model charges where applicable, application hosting and integration maintenance to the cost calculation.
AgentLine's published rates are monthly plans from $20 with included US or Canada numbers, call minutes and SMS; beyond the plan, $3.00/month per number, $0.12/minute for calls (per-second after a one-minute minimum; zero-second calls are free), and $0.02 per SMS in either direction. Check pricing and your account's allowances before budgeting. Runtime hosting and external integrations can add costs.
For the final choice, start with the phone-number API checklist and four-platform comparison. Pick the boundary that supports your required workflow and that your team can operate, rather than choosing from a universal speed or cost claim.