I realize it's been a while since this was asked, but I found that my s2 won't trigger my phone to switch forwarding on. This happens when I'm in an area that non volte isn't available. Turns out that the phone needs to send a command *86 something or other, but it's not compatible with volte. So my phone will actually drop service on my desk when I walk out of range, as it tries to send this command, turns off lte, and goes dead.
This shouldn't be a problem with the verizon or att version, but it is at least on tmobile if you're in an area that's ONLY serviced by extended range band 12 LTE.