From Cisco.com…..

MGCP gateway

Cisco Unity is unable to record messages for more than 50 seconds because the receive-rtcp timer times out on the MGCP gateway and the call gets disconnected. Disable the timer with these commands in the MGCP gateway in order to solve the problem due to disconnect of the call.

MGCPGateway(config)#no mgcp timer receive-rtcp

This command turns off the RTP RTCP receive timeout interval at the MGCP gateway.

MGCPGateway(config)#voice rtp send-recv

This command is used in order to establish a two-way voice path when the Real-Time Transport Protocol (RTP) channel is opened.

I’ll have to do this after hours and do some testing to see if the length of a voice mail will be extended.