Oh yeah... well Petunia, the phone works both ways...so do the cars and streets... You could counter with such ideas... you could also add some, like, why don't you write me any more mom? Did you make a list of what all you need done for you? OH? what are the neighbor's kids names, maybe I know them
Ok... a little more down to earth...are you the only child? If not, how often do your siblings call and visit her (regardless of how far away they have moved so they don't have to.)
I know it's a stress issue. A big one. Your mom's an adult, she makes her own choices. You get to, too.

What activities outside of her home is your mother involved in? If none, why not? She would then not be expecting you to entertain her... she needs outside activities. If she chooses not to spend her time for herself that's her choice...but to expect you to fill her time is not her choice.
Try other ways of giving her attention if you think it will help the situation and alleviate some of your undeserved guilt. YOU send HER cards, or fruit or such... If you must, then call her...but set the timer in the background and when it goes off (so she can hear it) just say OOOPS gotta go bye! And hang up

no other explanation necessary.
If nothing else, tell her, well I can't do all that stuff..it's the nature of PTSD and I don't expect you to understand it, few ppl do!