RexaThe short answer
The carrier is what stands between a voice agent and the phone network. It provisions numbers, routes calls, applies its own spam analytics, and enforces regulatory requirements. Carrier choice affects answer rates, audio quality on specific routes, and how caller identity is presented — none of which is visible in application code.
In detail
The landscape has several layers. Tier-1 carriers own long-haul infrastructure and interconnect with each other. Wholesale carriers buy capacity and resell it. CPaaS providers sit on top and expose an HTTP API, hiding SIP and interconnect agreements behind a developer-facing abstraction. Most voice AI platforms integrate at the CPaaS layer and inherit whichever underlying routes that provider uses.
Route quality is not uniform and is worth measuring rather than assuming. The same destination reached through different upstream paths can differ in audio quality, post-dial delay, and how reliably caller ID survives the journey. Grey routes — cheap paths that strip or rewrite caller ID — are a known cause of calls that connect but display wrongly at the far end.
Carriers also act as gatekeepers. They apply spam-analytics scoring that can label an outbound number before any recipient complains, they enforce identity-attestation requirements, and they can suspend traffic that trips abuse thresholds. Multi-carrier deployments exist as much for resilience against a single provider’s enforcement action as for price arbitrage.
How Rexa handles it
Rexa supports Telnyx, Twilio and Plivo as telephony providers, configured per workspace. Provider health is guarded by a circuit breaker held per tenant and per provider: five consecutive failures trip the circuit open, it stays open for a sixty-second cooldown, then admits a single half-open probe before closing again — so a degraded carrier is backed off from rather than retried into.
ProvidersRelated terms
PSTN
The PSTN is the global interconnected system of circuit-switched and packet-switched telephone networks that carries ordinary phone calls.
SIP trunking
SIP trunking is the delivery of telephone service to an organisation over an IP connection instead of physical phone lines.
DID
A DID is a telephone number assigned to an organisation that routes an inbound call directly to a specific destination without passing through an operator.
STIR/SHAKEN
STIR/SHAKEN is a framework of standards by which carriers cryptographically sign outbound calls to attest to the legitimacy of the calling number.