Learn Italian: È colpa mia, non ho potuto telefonare.

When a modal verb like potere (can/to be able to) is followed by another verb in the passato prossimo, it borrows the helper verb (auxiliary) of that second verb. Because telefonare (to call) uses avere (as in ho telefonato), the modal verb also takes avere: ho potuto telefonare.

Why is it colpa mia and not la mia colpa?

The phrase è colpa mia (it is my fault) is a fixed expression. Usually, Italian requires an article before a possessive word (like la mia casa), but this specific idiom drops the article and places mia right after colpa.

Could I say non potevo telefonare instead of non ho potuto telefonare?

Yes, but the nuance changes. Using the passato prossimo (non ho potuto) means that in one specific instance, an obstacle prevented you from calling. Using the imperfect tense (non potevo) describes a general, ongoing state of being unable to call, like if your phone was broken all day.

If I want to say 'I couldn't call him', where does the pronoun go?