Office Communications Server Team explained why simultaneous ringing is not applied for Response Group calls.
In this post we will 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.
Read more at source: http://communicationsserverteam.com/archive/2009/07/16/Response_Group.aspx