landline service

Must first reside at an address where ATT hardwired service is offered…

you can use the link below to see if and what internet service is offered

https://www.att.com/internet/availability/

If ATT Internet as a hardwired solution is offered then ATT Phone, which is a VOIP service connected to the back of the internet gateway, can be ordered.

https://www.att.com/home-phone/

Note that ATT Phone is an ADD ON product to ATT internet service, I would estimate the cost of internet and phone would be about $100 per month. Lower if he qualifies for ATT ACCESS account, depending on best internet speed available should be able to save $30 or more per month.

https://www.att.com/internet/access/

If by landline you are referring to POTS (Plain Old Telephone Service), which is the old fashioned copper lines that may be difficult as a lot of carriers and providers are phasing them out in lieu of VOIP as mentioned above. It's becoming too costly for the providers to maintain the copper plants (Central Office) let alone maintain the lines going to the house.