Write phone call scenes?

Hey. I'm busy writing my novel. The story is about a group of teenagers who did something terrible in their childhood, which they still suffer from today. However, the group has spread across Germany since the events, and everyone is now living their own lives (and dealing with the guilt they carry differently).

Then something happens in the present that stirs up the events of the past for the protagonist, whereupon he reconnects with another member of the group.

Now to the problem:

The group is reunited and meets late in the story, but the protagonist and another character work together much earlier to counteract the book's conflict. The two talk on the phone almost every chapter while physically separated.

Do I have to mention that it's a phone call in every conversation between the two, or is it okay otherwise, simply because it makes sense to the reader? If it's in the middle of a chapter, then of course something like "he dialed the number" is mentioned, but in the case I'm currently concerned with, the chapter starts immediately with a dialogue between the two.

Do I need to mention something like "he pressed the phone to his ear" or something else again? Or is it enough to just end with something like "he hung up"?
Because I'm slowly running out of ways to express myself without certain phrases seemingly being repeated.

Thanks in advance.