I called you, but you didn't answer your phone.

'I've been calling you, but you don't answer your phone' or 'I've been calling you, but you're not answering your phone'. If you mean that you called more than once, I think these sentences are better, because probably you would still like to speak to this person, so a progressive tense is more appropriate. If you only called once, you should say 'I called, but you didn't answer your phone' (no need to use the word 'you' twice).

I called you, but you don't answer your phone. (The above sentence can be used. It is correct when used to mean that you called once (or twice) but the person is always not answering the phone. The person still does not answer the phone now.) If you say: "I called you but you didn't answer your phone." This means that you called once but the person didn't answer that one call.