HIDING PHONE NUMBER FROM ISP

Not enough information. Where was the sharing taking place? Is her phone, or her account that is compromised? Two different issues.

Regards,

No its not, if your a netadmin at a local ISP i think you should know the answer?

There are ways of doing this. I know of two that work, but there are most likely more out there. However I agree with what saintychick mentioned, being a netadmin at an ISP you would first learn the go-arounds :P
More info is required, but yeah it sounds dodgy.
Like I used to write poetry and I used to give the poems to my english teacher saying "my friend" wrote them and that "they wanted to know what she thought". I was naieve and she always smirked when I gave them to her, it was after that, that I realised that they were written in my writing :O
SO if "this friend" of yours is experiencing an account violation, then "your friend" should contact their ISP. Account sharing via dial up is rare these days, but still possible; and the answer doesnt lie in hiding the number, rather than changing the password to something stronger.