Aspects relate to determining whether a constraint or other condition on a mobile device may be alleviated or better satisfied by switching from one communication network to another for a call (or setting up a call on one network in preference to another). A mobile device is operable to conduct a communication session over at least two different communication media, which have different power consumption characteristics. Where a remaining battery power level is constrained, a determination can be made whether using one network over another saves more than a threshold amount of power over another network. Estimates of battery life for each network can be made based on predictions of a length of the communication session, and a power factor calculated for the network.