Whether this rehab facility allows personal cell phones or not really shouldn't be a mystery. Just call them up and ask what is allowed in terms of outside contact.
And even if they do allow them (a very few actually do), there are almost always very strict rules about their use. What you've described is rarely appropriate behavior.
So, either way, this person is probably not taking his recovery seriously yet and any unapproved communication from you is not going to be helpful with his treatment.
I realize that avoiding personal contact with a loved one is one of the most difficult aspects of supporting an addict who is in-patient at a rehab facility. The professionals there deal with this issue often and they will typically provide the best advise along the way about how to manage it.
What you are experiencing is yet another example of how a substance abuse problem can negatively affect people other than just the addict himself.