Hi, I'm Nathan.Sorry to hear that you're having trouble with this but I'll do my best to help you.
On the ATT's base station, there is a button at the lower right corner that says ANS ON/OFF.
Pressing that once will turn the answering system on, which will play a default greeting (please leave a message) after 4 rings. That button will light up when it is turned on.
If you'd like help changing the number of rings or recording a personal greeting I can help with that too.
If you have any other questions, just let me know. Otherwise, please take a moment to rate my service as this is the only way I am credited for my time. Hopefully I have earned 5 Stars but if not please let me know what I can do to ensure you are satisfied. I also invite you to contact me directly with any future support needs at http://bit.ly/1QwLxGh and ask for Nathan.
Thank you, ***** ***** a nice day,
Your initial question was how to activate it, meaning to turn it on.
It will not show any numbers up to this point it it was not turned on previously.
Follow the above steps to turn it on, and then after people start leaving you messages only then will you see a number.
To listen to your messages, when you have some, all you have to do is press the PLAY button on the base.
If you are referring to a call option that is an automated advertisement which you may ignore.
I've given you very simple and specific steps to follow which will resolve your issue.
Do you need more help with this?