ATT Small Business System Model Number 974

This is what I think is happening at the bottom of page 79 of the manual found here this is the setting of home and local area codes it states the following:

If the call did not come from any of the area codes you programmed, (1) phone
numbers with 10 or more digits will automatically have a 1 inserted and
displayed before the number and, (2) phone numbers with fewer than 10
digits will be displayed without a 1

So, did you set up your home and local area codes? Also, do you have the area codes that have the double 1 in your directory? If not, the way I'm reading this if a call on the caller ID comes in and displays 1+area code+exchange+number will have an additional 1 placed in front of the 1 already there. I can't find anything in the manual as to how to keep it from adding the 1. You can also follow the instructions on page 82 on changing the dialing format in call history. You can call the number in the front of the manual to see if they know how to keep the extra 1 from being added. So, it's basically the format in which your carrier is delivering the CID. Don't know if they will send it without the 1 or not, you can ask. 

Found this on an old topic on the same issue.

So I solved this on my model 974 by changing the Home AC, under Area Codes (page 6 and 38) to just 1. Then I added the local area code under Local AC1. This instantly solved the Caller ID display issue for me.

That worked! Thank you, spoom2, for taking the time to find this fix and resolving years of frustration with this issue.

Glad I could help