MGCP or SCCP, these are the two protocols which comes to our mind when are asked to register a VG or FXS port on CUCM. But now Its not just MGCP or SCCP. We can register the FXS port’s using SIP protocols.

If below requirements met, you can register your VG/FXS ports using SIP protocol.

Requirements:
The FXS Ports for Supplementary Services feature is supported on Cisco VG450 Voice
Gateway and Cisco 4461 ISR. The FXS ports for Supplementary Services supports CUCM
version 12.5.1 SU1 or later with IOS XE 16.12.1 and above

Supported Features:
The following supplementary services are supported.
Call Hold
Call Waiting
Call Transfer (unattended/attended)
Call Forward no Answer
Audio Message Waiting Indicator
Call Forwarding Unrestricted
Call Forwarding Busy
Directed Call Park
Directed Call Pickup
Call Pickup Group
Three way Conference