When the Wideband AMR speech codec was activated in my 3G wireless network of choice a year or two ago I decided to lock my smartphone to 3G-only mode to prevent it being handed over to 2G when on the move and thus being downgraded to a narrowband channel. In the Cologne / Bonn area where I live this works great as 3G coverage in the cities and in between is excellent. In other words, even though only using 3G I haven't experienced call drops due to running out of 3G coverage during voice calls. But I have to admit coverage is not like that everywhere.
Obviously the countryside is often still not covered by 3G. No big surpise here. But when I recently visited a relative living in an area where a few towns with a population of around 150k people are very close together I was surprised how often I did not have 3G coverage especially between the cities. I quickly realized that my 3G-only setting is definitely not an option there. An interesting realization especially if you continue that train of thought in the VoLTE and SR-VCC direction…
I also track the number of active/monitored nodeB legs/links on my phone – once I get out of urban areas it is amazing how few links are actually monitored on my iPhone.
When you combine all of these considerations with the time taken for CS fallback on LTE if there is a call, then the implications are indeed interesting
VoLTE wont be an issue as 800MHz and 1800MHz should provide better coverage than 2100MHz 3G today. Also fast fading should be less due to OFDMA and wider channels.
I suppose having GSM helps, also with SRVCC – SRVCC to GSM is not too much different from SRVCC to 3G.
Of course, in this case any possible data transfer will either be suspended altogether, or be reduced to some tens of kilobits per second (if both the network and the phone/device support DTM).