When you are on a trip you are not "on call," so there isn't even a need to have your pager with you, broken or not. (When I leave for a trip I leave my pager at home.) If they wanted to get a hold of you they could have put it on the rids or had the crew give you the message if and when they called in range. Otherwise you just go with the original trip you were assigned. There is no way that any SD would have been upheld because you were not required to be at scheduling's disposal as you were already assigned a trip.