Select IP phone account for native dialer

I have two different SIP accounts: one is my favorite, but is only available from my home network. The other should be used from all other wi-fi networks. In case that I do not have any wi-fi connection, I would like to use the regular telephone service. I managed to setup both accounts as IP accounts in android (4.2, G Nexus, native SIP support). However, I can only define one of the IP accounts as primary. This means that I need to change the primary account manually whenever I leave or come home. Android only asks whether to use IP or not, but doesn't ask which IP account to use. Is it possible to change this behavior? If possible, I would like to use the native dialer and no SIP app.