This issue occurs with the Cisco 7960 IP phone when a 7914 expansion module is used.
In this setup, there is one line on the phone and one line on the expansion module. You can press the Speaker button to answer calls that come in on the line configured on the phone. However, when calls come in on the expansion module line, you cannot pick up the call after you press the Speaker button.
In order to answer the call, the user must press the Answer softkey.
Also when a sidecar line is in active call, you cannot press the Speaker button to hang up the call. Use the handset and EndCall softkey in order to end the call.
In both the cases, the phone display prompt changes from speaker to handset. However, the call is never picked up or ended. The root cause is that when you send the StationOffHookMessage, the phone sends the line button number instead of the line instance.
This issue is documented in Cisco bug ID CSCsa69644.
This issue is resolved in the phone load version 7.2(1).