Is there a way for 3G to not disconnect when connecting to WiFi?

WiFi is unreliable, I wouldn't trust WiFi for SIP calls, I've disabled WiFi within CSipSimple. Now every time I connect to WiFi, my SIP registration seems to go away, and I'm not receiving any calls at all. It's also unclear what happens when you do receive a call, unlock the phone to pick it up, and then the phone connects to WiFi (which it presumably didn't do during the sleep in order to conserve the power). Is there a way to make this work?