I know people who will not share their personal phone number for anything work-related, for a reference, anything at all really.
A colleague created a WhatsApp group for our department but one man refused to be on it, saying it was work and he didn't want his personal phone number being used.
I've known people refuse to be referees for jobs, housing, passports etc. because they don't want their phone number being given.
It's not something that bothers me at all really but everyone's different. Just wondered what people think?