When a user tries to forward a line in a group (for example, a group of six phones at extention 6000, shared line configuration), the call forwarding fails. An offnet number (or external number) is configured on the Cisco CallManager for forwarding all the incoming calls on extension 6000 to the offnet number.
Any incoming calls on extension 6000 should be forwarded to the offnet phone being configured. But in this case, whenever a call comes in on extension 6000, the call is forwarded and the offnet phone rings once before it drops the call. The configuration works in Cisco CallManager 3.1 but does not work with Cisco Call Manager 3.3(2)spB. The calls can be transferred from extension 6000 to any of the internal IP phones. However, if transferred to the offnet numbers, the call is dropped.
The call transfer to the offnet phones fails because extension 6000 is a shared line. When you forward calls for a shared line, all phones with this line must forward to the same number. Only a single phone on this shared line cannot be configured to forward the calls to an external number. This feature is currently not supported on the Cisco CallManager 3.3(3) and earlier.
For more details, refer to Cisco bug ID CSCdx61140 - 6624 unable to blind transfer offnet .
The workaround for this problem is to remove the shared lines from the configuration, and create lines on the IP phones that would forward out to the desired Public Switched Telephone network (PSTN) destination.