6S (which is an iPhone) or S6?
Regardless, there's probably something in the phone that's outgassing (plastic outgasses when it's fresh) that's being detected by the smoke alarm. (Or something in the phone is burning
very slowly (which, for your sake, I hope isn't the case). If it's an S6, and knowing Samsung's record, it's possible that it has a bad battery (which, in this case, should be covered under the warranty).
First try it in someone else's house, just to make sure your smoke detector isn't giving you a false positive. If it still sets the alarm off, bring it to your carrier and see if it sets off
their smoke alarm. If it does, they'll have a solution (like, maybe, replacing the phone).
Just make sure that you back the phone up first - see
Backing up an Android Device if it's an S6 (or ask in
iMore ask a question if it's a 6S).