too much interaction but a good idea if this is not possible to take automated effects.

having a user adjustable setting for when does the device switch to 3G and when does it switch back is the best solution so far.
But for incoming VoIP calls. If the switch happens in the same delay the user needs to answer the call fine but if it takes longer the call will be dropped. the call negotiation doesnt need 3G, the ringin doesnt as well.
Is it possible to keep connections alive or does it need to reconnect when an upscale to 3G happens? Does this depend on the setup how we control the hardware, is it protocol related or even a provider problem?

EDGE and UMTS happen within different channels the EDGE uses a time based routine known as TDMA and UMTS is using a code based routine known as CDMA. But thats only protocols the real hit comes to connection, only CDMA2000 and W-CDMA share the same duplexer mode (same connection) with EDGE. At that point it the host network is coming to play! If the negotiation of your host supports channel hopping and the used channels are CDMA2000 and W-CDMA you will be fine but if it doesnt you drop connection any time you switch channels. even if its within 3G technology and the provider doesnt support the hopping it will reconnect.

Example: you are somewhere with CDMA (HSPA) and you provider decided to use at the train-station another radio-station type as downtown (because of the amount of people or something) it could happen you drop connection and reconnect while entering to its radio zone, maybe you get more speed, maybe the provider gets more channels (users).

so the only chance I see doing it is to scale down the 3G device and keep connected, but edge is already fast enough to do most of the basic stuff, even VoIP should be possible (its not that slow).

As I am not a professional, please, proof me wrong if I misunderstood the basics of wireless networking and I will correct it asap.