How to redial second to last number called?

Depending on what service this is, you don't say, you can look at the call log online.  There is no way to redial anything other than the last number on landline phones. 

Thank you! In order to potentially view the log, do I make a new online account, or am I to somehow input our existing information?

You log into your (your mom's) MyAtt account and go to the phone features. She has to have digital service to do this it won't work on a traditional landline. 

What exactly is digital service? I am fairly sure it is a landline.

That's where you get your phone service out of the back of a modem instead of wired direct to a jack. 

Darn, ours is from a jack. This means I have no way to view the log, correct?

Correct

Thank you very much for the fast, quality assistance.

You're welcome.