'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?
1. Yes, exactly (well, 'not even once' sonds a bit off; it's more like just 'once' because perfective refers to a single action)
2. You were not supposed to GIVE ME A CALL (позвонила=once), you were supposed to CALL ME EVERY SINGLE GODDAM DAY!
3. Я не звонила потому что не смогла = 'I (repeatedly or continuously) didn't phone you as I wasn't (ONLY once) able to.'

...not even once... again implies multiple attempts, while what she's talking about was only one attempt -- that one in between the periods when her cellphone was 'lost'.


PS. My afterthought was that your translation of the Eminem's 'Letter to Slim' / Stan wouldn't hurt to get the difference between imperfective and perfective aspects (that exerpt from his song, the one with "We waited in the blistering cold for you and you just said 'no'... ")