My understanding is that it's Kuroki's (the closeted seme) phone because of the mention of the handcuffs and the fact that he had them from the start. I think he was contacted by a former lover for a hookup and is saying he can't because he has a bf (Kiumi) now.
In case anyone's curious about the text convo:
- Haven't seen you lately, let's hook up.
- Sorry I've got a boyfriend now. Dating (?) only.
- No way! Congrats! You still have my handcuffs btw.
- I'll give them back.
- It's okay! You can have them.