I would ask T if she meant for you to have it.
I think each situation is different.

My T volunteered her cell phone number a few months in.
I thought I would NEVER use it. I see her weekly, I'm relatively stable.

Yet in the last year or so, I have begun to text her between sessions. Usually once, at times twice. Many times not at all. I hope for and get quick responses back. Just reassurances to my vents about life stresses usually
I've never wanted to call the cell.

She's told me she's glad I send her texts. Something about me feeling I can ask for help, a problem for me at times!