The difference in bandwidth between 3G and 4G should not be an issue when using VoIP unless your phone is also downloading a huge amount of data while you are on a call. There are many other factors involved that have a greater impact. How the operators network is set-up, is the biggest. I recently used the 3 network in Italy (3G), very few issues, and when i checked my phone, the majority of times it showed a green dot (registered). In England on Vodafone, (3G) i found there was an issue registering unless i used the tunnel. As there is no service on the Tube, the phone would have to re-establish a 3G signal and re-register much more often that in Italy, many times, when I checked that phone, the dot was red and I had to manually register, which always worked.
So there are probably three basic factors at play... your particular handset, and how it "handles" things, the Operator you are on, and how often you loose a signal and/or move between base stations.
