Mobile Client only rings for half a second for an incoming call
The Mobile Client rings only half a second when alerting an incoming call. However, after that the call continues ringing only on the Cisco IP Phone. The very next call could ring both the Mobile Client and Cisco IP Phone as expected, with no intervention or changes. Issue happens seemingly at random.
In sending the initial alerting tone to the mobile client, if the mobile does not respond within the SIP Dual Mode Alert Time period (default of 1500 milliseconds), CUCM will send a CANCEL to the mobile client which stops offering the call to the mobile client; and continue alerting the other devices associated with the DN dialed.
In to Cisco Unified Communications Manager admnistration navigate to System > Service Parameters > Cisco CallManager (Active)
Scroll down to Clusterwide Parameters (System - Mobility)
Increase the SIP Dual Mode Alert Time to a larger value; i.e. 3000 milliseconds or greater
In later versions of CUCM, you can disable the SIP Dual Mode Alert Time. Follow ehnancement CSCtn92569