No, it is not a bipolar symptom per se - read the DSM. There are 1001 reasons for why he might sometimes not respond to calls.

Not answering the phone and not responding to any incoming traffic can be a symptom of depression when a person isolates himself from the world (which often instigates a downward spiral, because more isolation leads to more depression), but you would need more than a day of not responding for that to be the case, and, you would need other signs of his isolating himself from the world. An occasional blunt response or an occasional non-responsiveness for a day or two - not enough information to make any pronouncement.

You need to read up on bipolar - it is indeed an episodic illness, as you said, but episodes have a certain duration to them. A day or two of inconsistent behavior would not be enough to say that he is having an "episode", unless the behavior is so egregious that it lands him on a locked unit. Read the DSM - it talks about what qualifies as an "episode", with emphasis on minimal duration. While reading the DSM would not make you a doctor or enable you to diagnose your bf, it would at least provide you with some background knowledge so that you would not look for "signs" where there are none.

On a more practical note - when he does not respond to phone calls, have you tried also emailing him?