Calling again at this time, with the last being at Christmas, is not stalkerish. IMHO, you are allowed two attempts per visit, when such visits occur only once or twice a year.
Here is a script:
Hello, Mrs. X. This is Y, a friend of your son's. I understand he is in town right now and I would like to get in contact. Unfortunately, the last time I was in contact with him he didn't have his own number yet and gave me yours. Do you have a current phone number I could use to get in touch with him? I appreciate your help.
If she mentions the Christmas thing, just go with the flow (if she apologizes again, say its OK, there are always other opportunities; if she asks if you're the same person, just say yes, and so on.)
Phone calls to people you don't know well to ask for what is essentially a favor are, by nature, uncomfortable. But that doesn't mean you shouldn't make them. She should be happy to help you; most parents enjoy knowing their kids have friends who want to reach them.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).