The problem is related to CSS in gateway and CCM.

How to Pick Up Calls that Belong to Another Group:

Group Call Pickup allows you to answer incoming calls that ring on a Cisco IP phone that belongs to a pickup group other than your own. To use this feature you must know the pickup group number to which the ringing phone belongs.

When an incoming call is received on an extension that belongs to a different group than that of your IP phone, follow these steps to answer that call:

Go off-hook on any available line appearance.

Press the More softkey and then the GPickup softkey.

Dial the Group Call Pickup number.

Press the Answer softkey.