If you need to express urgency, send me a text message along with a telephone call; but please don't ring my phone off the shelf.
If I don't pick up the phone when it's ringing, I'm not going to pick it up when it rings again. Fair enough, I might not have credit on my phone, and be unable to ring back. But send me a text message; let me know if it's urgent; give me the chance to decide whether or not to pick up on your next ring.
I'm not constantly accessible. I don't want to be; that's why I am not. That's why I don't always carry my phone with me; that's why I don't always pick up.