Compare SIP and WebSocket architectures for connecting phone calls to AI voice agents - pick the right fit for latency, cost, and deployment speed.
These are the two fundamental architectures for connecting phone calls to AI voice agents. The choice is not about which is “better” - it depends entirely on what you are building, your platform choices, and your priorities between deployment speed and total ownership.
SIP Trunking
Telephony industry standard - robust, enterprise-ready, and mandatory when you need call transfer, PBX integration, or managed platforms like LiveKit, VAPI, and Retell AI.
WebSocket Streaming
Developer-native path - highly cost-effective at scale, more direct, and ideal for custom AI pipelines built with Pipecat or bare-metal code.
The two approaches operate at completely different layers of the stack. SIP is a telephony-layer protocol. WebSocket streaming is an application-layer transport.
Fetches your webhook, receives VoiceXML stream directive
3
WebSocket (wss://)
Direct TCP connection established to your server
4
Your Server
Receives base64 µ-law audio directly from Vobiz
5
AI Pipeline
STT → LLM → TTS logic driven entirely by your code
6
WebSocket (back)
µ-law voice audio sent back over same socket connection
Important architectural nuance: These two architectures are not completely mutually exclusive. You can use a generic SIP Trunk provider to route a call to Vobiz, and then use a Vobiz VoiceXML <Stream> directive to pipe that exact call to your custom WebSocket server. SIP handles the initial routing; WebSocket handles the audio layer.
The Vobiz channel rate is identical for both paths in spirit - the difference comes from whether you add a managed AI platform layer on top (SIP path) or own the pipeline yourself (WebSocket path). All pricing below is in INR.
You build the pipeline yourself. This is the key saving.
STT (e.g. Deepgram)
Direct API rate
Pay STT provider directly
LLM (e.g. GPT-4o-mini)
Direct API rate
Pay OpenAI / Anthropic / Google directly
TTS (e.g. Cartesia / ElevenLabs)
Direct API rate
Choose your TTS provider
Server compute
Cloud infra
One process per concurrent call
Vobiz WebSocket rate: ₹0.65/min + ₹500/month per number. Total = Vobiz rate + direct AI API costs only. No platform markup.
The bottom line: Under 50,000 calls/month, the platform premium is often worth the saved engineering time. Above 50,000 calls/month, owning the pipeline (WebSocket path) pays off significantly.
SIP has lower audio transport latency than WebSocket streaming. SIP uses UDP/RTP - a fire-and-forget protocol that never retransmits dropped packets, keeping audio delivery strictly real-time. WebSocket runs over TCP, which guarantees delivery by retransmitting lost packets - useful for data, but a source of jitter for live audio on poor networks.If latency is the only factor you care about, SIP wins. But latency is rarely why developers choose WebSocket streaming. They choose it for the ecosystem - direct access to AI frameworks (Pipecat), raw STT/LLM/TTS APIs, full pipeline control, and lower cost.
