My experience today of Supervoice using TiT today wasn't so great.
First time i tried calling (in an area i tradtionally got no signal in the past. Ever), albeit with 1 bar of 4G, i got through to the other person who sounded robotic and who couldn't hear me and like other above have said, it defaulted to loud-speaker.
I could use data just fine though.
Next, i was in the centre of Guildford, where i couldn't get any 3G, was hoping for 4G Supervoice to kick in, being that i believe the 800Mhz mast is about 1 mile away and...nothing.
After about 5 mins of trying to make a call i noticed i had gone over to 'R' above the signal icon on my phone and was able to call.
Using a Z1 Compact, with custom (non Three) firmware.