Are Jitterbug flip phones compatible with AT&T's Network?

Hi,

Are Jitterbug flip phones compatible with ATT Network?  I have 90 yr old and 87 yr old in laws living with us and they like flip phones and only use their phones to make and receive calls.  No txt, internet, pictures, etc.  Just calls.

I purchased the phone that is listed on the ATT site 6 months ago and not only is it not working anymore, but she can't hear when people call.  The volume is not loud at the full volume level.

I need another option other than what ATT has listed as the option for a flip phone.

Thank you in advance...