Is it possible? My 6+ shows the 5S in the Text Message Forwarding settings, and when I turn it on for the 5S it says to enter a code that shows up on my 5S. On my iPad Mini and Air, the code displayed immediately, but on the 5S, nothing appears? iMessage is activated on the 5S. I had to take my SIM out of the 6+ and put it in the 5S for that to activate, but once I did that it worked. I just wonder if SMS Relay is not available when using an old iPhone as an iPod Touch. Note that I did a fresh restore on the 5S and set up as a new phone before doing any of this, and handoff is working no problems on the 5S.