'Я не позвонила потому что не могла.'
'I did not phone you (at all, not even once) because I was (repeatedly or continuously) unable to.'

'Ты не позвонить должна была! Ты должна была звонить мне каждый божий день!'
Not sure how to translate this... 'You couldn't (at all, not even once) phone, (as you) should have! You should have phoned (repeatedly) me every goddam day!'

'Я не звонила потому что не смогла.'
'I (repeatedly or continuously) didn't phone you as I was (not even once) able to.'

Is that what's going on here?

I'm glad you (they) resolved this one; I think he was just jealous cos he got left at home all on his own...

I get what you say about the perfective-imperfective mutation in imperatives, that seems very logical to me. So maybe I'm getting a general idea of aspect. It kind of feels like I am, anyway.