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Technical Deep Dive 6 min read
Beyond $0.10/min: The Architecture of Sub-Second Latency in AI Sales
Alex Chen
Senior DevOps Engineer • Feb 18, 2026
The Latency Imperative
In AI voice sales, latency isn't just a metric—it's the difference between a conversation and a robotic interrogation. If an agent takes 2 seconds to respond, the prospect hangs up.
At ASOC, we obsessed over this. We didn't just wrap an API; we built a hybrid voice stack designed for sub-second performance.
The Stack: Retell vs. Vapi
We leverage the best of breed:
• Retell AI ($0.12-$0.16/min): Our workhorse for rapid, stable, high-concurrency outbound dialing. It handles the telephony edge cases beautifully.
• Vapi AI ($0.10/min): Used for highly custom, developer-led flows where we need granular control over the function calling loop.
The Architecture
graph TD
A[Apollo.io Signal] -->
Webhook
B(ASOC Core)
B -->
Select Agent
C{Router}
C -->
Standard
D[Retell AI SDK]
C -->
Custom Logic
E[Vapi AI SDK]
D -->
SIP/PSTN
F((Prospect))
E -->
SIP/PSTN
F
F -->
Voice Stream
G[Deepgram Nova-2]
G -->
Text
H[GPT-4o]
H -->
Function Call
I[Calendly API]
Function Calling for Closes
The magic happens when the AI needs to take action. We don't just prompt the LLM to "try to book a meeting." We give it tools.
Using GPT-4 Function Calling, our agents have direct access to:
1. `check_availability(email)`: Pings the prospect's calendar (if public) or our SDR's calendar.
2. `book_slot(time)`: Curls the Calendly API to lock in the meeting.
This reduces the "administrative gap" to zero. The meeting is booked before the call ends.