Usually men don't like to talk on the phone. You don't say his age - but if he's not a teenager then i would say he does not normally like talking on the phone.
I think you knew the answer to your question before you posted it but just wanted to talk about it. Yes, it's drug related. If he's an active addict, everything is drug related. Whether he is buying or selling i don't know, but you would know if you lived with him.
Remember having one land-line in a home? When the phone rang, anybody could answer it, and the call could have been for anyone in the house. My regret with cell phones is how it distances people. My son just got married, and because he and his wife have cell phones, they do not get a chance to talk to each other's friends (even just to answer the phone). Even if I call this son, because it is his cell phone i never get a chance to say "hello" to his wife because she is never answering his phone...
I can see why addicts love cell phones because it helps them be secretive ....