Actually, one can "telephone" a friend, but this is less commonly used than "phone" a friend. Both are used as verbs and both are correct English. So this phrase can be translated as, "Sorry, I must telephone," but it sounds a bit awkward or formal. That answer was accepted, BTW.
I entered 'I must telephone' and this was accepted but I am not sure what I have said. If I am sorry about something, I suppose it is that I have to interrupt a conversation to answer the phone. So maybe "Sorry, I have to answer this (cell phone) call." or 'Sorry, I have to pick up the phone.' Does the french sentence in question adequate for either of these?