Can someone please explain how to use the phone?!
The only way to use the phone, currently, is for texting Lisa or calling Mia.
Calls to Mia are timed events. The first one and the third one can't be missed, so don't worry about it. The second one is a matter of the MC being in her room at the time when the call is supposed to happen, so pay attention to the time on that day.
If you're asking how to use the phone to text Lisa, then search this thread for "call Lisa", and follow those instructions, since they haven't changed. The only thing that's changed is that the MC now texts Lisa, pretending to be someone else, instead of calling her.