I will not claim to be an expert on the matter, but it seems to me that a handover shouldn't be that difficult to do completely through software. As long as the device (phone) is capable of using both WiFi and cellular connections simultaneously, why couldn't the phone detect the WiFi network and make the VOIP connection in the background, then switch over when ready. In the other direction, it could detect the WiFi signal and switch to cellular when it begins to weaken.
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I will not claim to be an expert on the matter, but it seems to me that a handover shouldn't be that difficult to do completely through software. As long as the device (phone) is capable of using both WiFi and cellular connections simultaneously, why couldn't the phone detect the WiFi network and make the VOIP connection in the background, then switch over when ready. In the other direction, it could detect the WiFi signal and switch to cellular when it begins to weaken.