Being that I'm usually at work and talking to a stupid customer that is pissing me off, I usually just mute the phone and swear at them. *This backfired once when a coworker was using batteries for his phone (I use a power supply) and they wore out. The customer heard everything he said!! <edit> Oh, here's a stupid customer right now!!!
To relieve my anger, I might kick something like a table, and accidentally hit my shin on it. Then, I'll be so overwhelmed with pain that I would forget about my anger.