Since a phone call in early December when my mother started speaking incoherent gibberish and I was scared she had had a stroke, I have phoned every day. It's not cheap, but as I don't see her very often, I don't begrudge the time or the costs. Anyway, most times she remembers to offer to hang up straight away and call me back, as she has a flat rate.
She laughs that she "knows I am monitoring her", but I find it helps not only to reassure me but to keep her brain active, (even though we have the same conversation every evening) and her voice strong. (She's 95)
As for my boys phoning or contacting me - they have busy lives and, as with most people here, they can go for weeks sometimes without any contact - "No news is good news" is my motto, but I never feel they're completely out of touch.
The older two, now that they are fathers themselves, have a different attitude to contacting us and realise that we want to know they're OK and to hear their news. They don't mind a spontaneous call every so often.
If your relationship allows, Jennypops, why don't you just fix a regular time for calling? If you only talk every second Sunday and take it in turns to call, that only makes one phone call each a month. Finish off with a reference to the next phone call and how much you look forward to hearing from her then (with a reference perhaps to something specific that will be happening in the mean time).
Keep your pecker up! and I hope things get better for you.