It's rude to ignore someone's phone call.
Lee is in a bad mood because of an incident at work and his friend, who he knows needs some advice, is calling.
Lee wants to be left alone for the time being.
Lee picks up the phone to tell his friend he can't talk now and to wait until he calls back later.
Lee just lets it ring and presses the volume button to silence the ringtone.
Lee's friend decides to wait before deciding anything until he returns the call.
Lee's friend makes a reckless and poor decision without the benefit of outside counsel.