This post explain why it’s important to not ring the user’s additional number for Response Group calls, and how that is accomplished. The main goal of the Response Group Service is to route calls to individuals. Enabling simultaneous ringing means that a user has set his forwarding settings to ring an additional phone number. Doing so means there is a chance that the call can end up on a voice mail system (for example, the voice mail of the user’s cell phone). It would not make sense to have calls routed from a Response Group to someone’s voice mail. This is the reason why simultaneous ringing is not applied for Response Group calls. Disabling simultaneous ringing is achieved by setting a special header in the SIP INVITE sent by the service to OCS. OCS will then interpret this header and not apply the forwarding settings of the user.
Full Article: OCS Blog