Core Issue
When a plain old telephone service (POTS) phone on the Private Branch Exchange (PBX) calls an IP phone on Cisco CallManager Express, the Cisco CallManager Express IP phone rings but the PBX POTS phone does not hear the ringback. If a Cisco CallManager IP phone calls a Cisco CallManager Express IP phone, the Cisco CallManager Express IP phone rings and the Cisco CallManager phone hears the ringback.
Resolution
In order to resolve this issue make sure the Media Termination Point (MTP) on the Cisco 3825 Gateway defined in the Cisco CallManager is unchecked.
Refer to Troubleshooting No Ringback Tone on ISDN-VoIP (H.323) Calls for various "no ringback" issues.
