Let your mom know she has no reason to be scared - the caller is a scammer trying to scare people into paying money they don't owe. Read what was posted above - the IRS scam is pretty common nowadays. You may want to look at blocking the number on your mom's phone, or buy a call blocker.
I really callled them back and told them I actually work for the irs and asked why I was receiving this call??? Of course they hung up... It only worries me because they knew my name.