Telephone interconnection is working one way? Phone user can call radio subscriber, but radio subscriber cannot speak to phone user by pressing PTT (Channel busy)?
IP Site Connect topology
First of all. Make sure that codeplugs of both repeater and subscriber unit are newer than R01.08.00.
If first is true, then most probable answer is wrong CPS/radioserver settings.
This situation can be caused by mismatching numbers for Gateway ID in CPS and Peer ID for IPSC system. When configuring phone system in CPS, Gateway ID must match the Peer ID for used IPSC system.
Capacity plus / Control stations topology
Phone calls are not supported natively in topology that is utilising Control stations. This fact renders phone system configuration in CPS obsolete, as telephone interconnection is organized by means of radioserver. However, subscriber's radio must have the ability to interrupt ongoing voice call of control station to participate in phone call. Control station itself should be configured to allow being interrupted. If these settings are not done, radio subscriber will be unable to talk.
1) Check CPS settings of control station, make sure it is allowed to be interrupted
2) Check CPS settings of subscriber's radio. Make sure it is programmed to interrupt control station.