My Mom got a phone call from MY phone and it was TURNED OFF!!

I am currently travelling in Europe and my prepaid AT&T phone is turned off as I do not have service outside of the US.  I have it physically turned off!  It is paid up and I will turn it back on for use when I am back in the US in a couple of weeks.  But this morning my Mom got a call on her landline and it says it was from MY phone!!  They did not leave a message, but I am wondering how in the world this could happen.  Do I need to worry about my phone being hacked?  I checked the service record and there have been no outgoing calls showing from my phone.  She called my phone and it went straight to voice mail, and that shows up in my record.  The only  way my phone is associated with my Mom´s landline number is that it might be on record with my credit card  (AT&T Credit Card!) as this was my landline phone at one time, but for years I have only used my mobile and do not give out her number.  Do I need to be worried?  If so,  what do I need to do?  Thanks for any response!!