We have an installation with Lync 2013 where all users have enterprise voice with their own assign phone number.  Some users are members of a responsegroup, and when they are "connected" to response group they want their outgoing phone number to be the phone number assign to response group.  They can choose this for each time they call, but they want this to be default.

Is it possible to make this default?

You can't do that. Lync will use the voice policy that assign to the user and then send the users own DID unless there's no CLI suppression is configured in route level. You could try put a number manipulation rule in to the PSTN gateway to look at the user DID and change that to the number that you want and send it to the PSTN. But, that would be a static change. Users won't get to select what number they want to send out.