SPOILER? I don't THINK this is spoiler but really, everything about a movie gives a little something away so I wouldn't read if you haven't seen it.
In the scene with the M girl in the car, before they have sex, he goes to a phone booth is calling someone and it rings and rings and he's very frustrated and bams the receiver in anger -- before returning to the car.
What was that about? Who was he supposedly calling? Not Giorgia, as she wouldn't have a phone number. Was it the prostitute? Who -- I had no idea when watching and it kind of bothered me.
I haven't watched the film in a while, but I don't recall him calling anyone - I thought it was just the dial tone. He's not calling anyone, was my interpretation - he just needed to get away from Mirella's affection and attention. It was too much for him.
I don't know whether you maybe viewed an abridged version. The version I saw was 6 hours in length. In my version it was quite clear who Matteo phoned. Shortly after he leaves his mother's house, he is troubled by the memory of his encounter with Mirella (at the police headquarters, where he doesn't give her the chance to reveal that she is pregnant). We then see an alternation of two things: Matteo in the phone booth, and Mirella lying on her bed and not answering the phone ringing. This happens twice. The second time, she overcomes her anger and decides to pick up the phone anyway. But she misses him by a second (She calls his name; she knows perfectly well who called her at that early hour of new year's day.) Matteo gives up and goes home. We know what happens then. No wonder Mirella is extremely shocked, when later that new year's day she comes to his appartment, and finds out what he did. (This is reported by the landlady, who tells all about it to Matteo's elder sister, who does not really hear what she tells about this fine looking young lady, because she is irritated by the woman's idle chatter and overcome by grief).
That was a different time. The first time he was in the car w/ Mirella, he needs to get away from her. So he fakes the call, that he has to go to work, to get away. I think he is feeling smothered.
Smarr is right. Matteo does not dial any number. When he gets back to the car, he starts lying to Mirella that he has an urgent work assignment and must take a plane early the next day.
It was intentional that he actually faked calling someone because he was unsure of his own feelings he basically don't know what to do next. From earlier part of the movie, he had an encounter with a transsexual prostitute- giving the hint that he is into that type of lifestyle. I would even think he's gay even from the start of the movie.
It just happened that M got pregnant with that one particular moment they spent together- and actually made the story a lot better.