I remember that in Kotomi’s route, there’s a scene where Tomoya uses his cellphone to message Kyou and them.
Other than that, I’m pretty sure they don’t use it much, or show it much. There’s also one scene in After Story where a phone is used, a house phone, not a cell phone.
There’s also the dorm phone that was used when Mei calls Sunohara for the first time (and times later on).
Your best chance at finding someone using a phone would be either Kyou, or Kotomi.
EDIT AGAIN - In the Angel Beats 1st Beat Interview that Kaza posted not-too-long ago, Jun Maeda was asked about the characters and which ones had a backstory and didn’t. Jun Maeda said that Sekine (along with 3-4 other SSS members) was at a concept stage. In the VN Database for the 1St Beat, it says that Maeda is responsible for Sekine’s route ? Was this just a false info ? (Hoping for someone who actually has played the 1st Beat to answer)