voip network hopping

i'm curious if there is any spec in the voip protocol suite that allows one to maintain a call while changing networks? what i want to do is setup a softphone on an android phone. however, this won't work very well if i can't switch from wifi -> 3g -> wifi (i doubt wifi -> wifi is possible because of hardware limitations). my alternative is to get asterisk to put the call on hold and play an automated message if a reconnection (and unhold) takes >1 second. i would expect that ip telephony would allow for some type of hopping though, i just don't know what to google for?