With an IOS voice gateway that runs MGCP and Cisco CallManager, an audible squawk is heard for a few seconds, followed by a normal media cut-through when answering a call on a phone connected to an FXS port that has Calling Line ID (CLID) enabled
When using a voice gateway that runs Media Gateway Control Protocol (MGCP), there is a high pitch tone for several seconds after the first ring followed by a normal media cut-through when a Foreign Exchange Station (FXS) line answers a public switched telephone network (PSTN) call. If you answer during the first ring, you can hear this high pitch tone on the device. This causes problems especially on a port that goes to a paging system. The paging system picks up on the first ring, and the beeps are heard on the overhead speakers.
This issue is documented by Cisco bug ID CSCsa97827.
This symptom is observed on all voice gateways that run Cisco IOS Software Release 12.3(14) or a later.
Downgrade the router from Cisco IOS Software Release 12.3(14) to any of these releases:
Cisco IOS Software Release 12.3(11)T5
Cisco IOS Software Release 12.3(11)T6
Cisco IOS Software Release 12.3(11)T7
This problem is also resolved in the resolved in the 12.4(5a) IOS version