What legal steps should be taken in regards to this dilemma?

This is rather complicated and long, so I thank all who take the time to read this in advance.

A while ago my mom received a collections notice in the mail addressed to her for an overdue cell phone bill from a provider she’s never had before. At the time, an aunt of mine (B) was living in the apartment complex where my parents reside. She and my mom’s first and last names are the same, but their middle names are different (they married two brothers). My mom went to go talk to her and asked her if the bill was hers. B said it was, and my mom told her to call the company and straighten it out and left. It’s obvious now that B didn’t do anything about it (called nor paid it), as it showed up on my mom’s credit score when my parents were talking to a realtor about buying a house (they are still going through the process). The realtor told them that she did not advise them to do anything about the issue yet, as it would complicate applying for a loan. Anyway, my mom is still getting notices about the unpaid bill, even though she has called the company to tell them about the issue. So, finally, here is my question. How do we handle this issue legally? We don’t know where B lives anymore because she moved. And is it true we should wait to take any legal action until after my parents get the house stuff over with? Needless to say this has been a pain in our necks and has even messed things up with the parent student loans my mom took out to for my college expenses.