I am using it with fido. got a prepaid nano sim with $10 voice. You can get data as well I think.
You can only use it to forward calls from your phone to the gear if you disable voicemail though which sucks.
I am doing this just to try it out. You have to call fido to setup the forwarding instructions.
The calls do get forwarded when the phone is out of range but the gear s does not recognize that forwarding is on and keeps asking you to check it so I just press cancel. Its a neat feature but not yet properly integrated. I am hoping that when the s6 comes out, fido or others will fix this. Basically, they need to enable voice mail as active with conditional forwarding at the same time. Currently its one or the other. Plus likely they need to adopt some Samsung infrastructure.
Actually, it is usable! I had Fido disable Voice Mail. then setup call forwarding as per the instructions. It works but as I said, the phone does not know it is remotely connected. Then I had Fido setup Voice Mail again. Now, when I turn off bluetooth (akin to having the watch out of range of the phone), after about 30 seconds, the phone connects remotely to the watch, perhaps through wifi, not sure, then a few seconds later it says call forwarding is enabled! I called my phone from our land line and it went straight to the watch, no delay! Turned bluetooth back on the phone and it reconnected to the watch and calls came through both. So the key is to disable voicemail , enable call forwarding on the phone Gear settings and watch settings, then have fido turn on voice mail again. This was at home where I have wifi connection. I hope that this works where there is no wifi. Otherwise, its limited.