With zap channels a group can be specified so the first available channel is used, for example, the b channels of a PRI can be assigned group=1, and the dialplan can specify dial zap/g1 to indicate a free channel in group 1. How do you achieve the same thing when the channels are SIP? More specifically, lets say * is registered with 4 different SIP providers and you want to make an outbound call using any one of the providers that is not already in use. Would you have to set the dialplan to go through each one until it can find a peer that accepts the call request, or is there a smarter way similar to dial/g# with zap channels?