Confusion with calling mobile

evening all,
question. my son is using my partner’s old iphone 5c. when anyone calls him my partner’s phone rings and same other way around. Obviously he is using his sim etc… why is this happening? how to i ‘turn it off’?

Are they both using the same iTunes/iCloud account?

The only thing I can think of is that the old 5C is still logged into your partners old account and hand off is doing it’s thing.

I’ve seen it work bewteen iPad and iPhone but never between phones…

yeah, seems you guessed right. somehow had calls from the 5c checked on the se! fixed now, thanks