I have never experienced a Therapist allowing this but I know some do. I would never do it if I wasn't told it was ok and if I was told I could contact them, I would try very hard not to unless it was a crisis of some sort and even then I would probably seek other support.

However if you have been having issues and your therapist would rather you contact them than suffer in silence then it might be a good idea to follow their suggestion to ring them