Did some tests. I am using 6s while my wife 6, both on 9.1.
If both 4G set to data & voice, I make voice call, my wife phone will ring almost immediately, but on my phone the calling screen will appear only ~2s after.
If mine is set to data only while my wife remains data & voice, make call from mine, calling screen appear faster, my wife phone ring slower than on d&v.
If both set to data only, nake call from mine, calling app appears almost immediately but my wife phone ring a lot slower, the slowest among 3 tests.
If both 4G set to data & voice, I make voice call, my wife phone will ring almost immediately, but on my phone the calling screen will appear only ~2s after.
If mine is set to data only while my wife remains data & voice, make call from mine, calling screen appear faster, my wife phone ring slower than on d&v.
If both set to data only, nake call from mine, calling app appears almost immediately but my wife phone ring a lot slower, the slowest among 3 tests.