TAS

The TAS is where the actual phone features live. SIP and the CSCFs get a call connected; the supplementary services people expect — call forwarding, call waiting, call hold, conferencing, caller ID presentation and restriction, barring — are implemented in the Telephony Application Server. The S-CSCF routes the SIP dialogue to the TAS based on the user's filter criteria, the TAS applies the service logic, and the call continues. For 3GPP's standardised telephony service this is the MMTel application server.

Where it shows up on the job: when a VoLTE or VoNR feature misbehaves — call forwarding not triggering, conference legs dropping — the TAS is usually where you go digging, because that's where the feature logic and the per-subscriber service settings are enforced. The same TAS typically serves both VoLTE and VoNR, since the IMS service layer doesn't care whether the access underneath is LTE or NR; only the radio and the bearer path differ.