IMDB page for Scream posted:

Why is having a cell phone suspicious?

Billy is suspected of being the killer by the cops and Sidney, largely because he has a cell phone. Sheriff Burke even asks what he is doing with one. This may confuse modern audiences who live in a world where you get them for free with a plan and can literally find them in your local 7-11. However, you must look at the time the movie was made. Back in the mid 90s cell phones were mostly a luxury item not readily available to everyone. It would have been rare, though not entirely suspicious, for a teenager to have a cell phone.