In August 2020, Doppio accidentally acquired a new ability from Mana. His intent was to contact the Boss, but instead he gained the power to use small objects or living creatures as phones. It's essentially a long-range communication ability that works as follows:

Should anyone try to call Doppio via phone, he will be able to receive the call anywhere he is, as long as there is anything he can conceivably use as a receiver.

If the receiver is not, in fact, a real phone, he will not be aware of this fact.

The other person DOES need a real phone. And probably Doppio's number.

Conversely, Doppio can also call anyone from anything he can conceivably use as a phone, as long as he has their number.

Naturally, the person on the other end does need a real phone in order to answer.

Also, when he gets a call, he has to make ringing noises. Somehow, that's just how it works.