If someone buys you a phone and gives it to you as a gift do you have to return it if the demand for it back? I’ve had it for like four months

With limited exceptions, an unconditional gift (i.e. I give you the phone absolutely w/o any conditions v. giving you the phone "as long as you keep your g.p.a. above 3.0" or "as long as we are together") is irrevocable.  The main exceptions are gifts given in contemplation of marriage, and gifts given in contemplation of death (i.e. I give you my house believing that I'm about to die from a gunshot, but the doctors save me).