Well Mech, You are in a bit of a bind. Chances are if the person you are trying to reach is stressed about work issues, he/she will blow you off at this time of year. People from the outside can only call certain phones and it’s somewhat discouraged. It would not be a normal thing. On the other hand if they are desperate for help you might have an in.
I think the person who gave you the phone number should give you the email address and have you send an email and some warning that you are going to call.