I'm going to ignore my girlfriend's calls for three minutes while I call my friend.
Why is it three minutes specifically? Are you busy?
Because it's three minutes of my time and I can choose to spend them with whomever I want.
It isn't right to ignore a partner contacting you when you have the opportunity to answer his or her calls. Even if it is only three minutes, it still causes frustration and annoyance to your girlfriend when she can't reach you. You should first answer your girlfriend's call and then contact your friend or otherwise it would be rude and not proper etiquette.
It's fine. She can get pissed all she wants but that's the price she has to pay.
It is not nice to play games with your girlfriend like this. If she wants to talk then you should pick up the phone and listen. She is your girlfriend for a reason, and it is your choice to be with her.