Fastfred, I know what you mean, but I don't know what you mean (because I don't run a one-man shop like you do), but perhaps I can offer some helpful suggestions.
One thing you can do to reduce the stress level by a huge percentage (I'm basing this on the scenario you laid out in your OP) is to let those phones work for you with some creative outgoing messages...
"Hi, this is FastFred and I would LOVE to talk to you right now, but as you can imagine I am earning my nickname by working to finish up someone's car as QUICKLY as possible. Leave a message, and the next time I roll out from underneath it and wipe my hands off, I'll call you back. I hope you understand."
Triage with a smile; when you roll up on a similar situation, prioritize who you need to speak to first (it would be the UPS guy), smile at the two customers, say "Lemme sign for this and I'll be right with you," and then after you've gotten him on his way, ask, "Who was here first?"
All this time, neither phone is bugging you because those ringers are OFF.
The customer who wants to know if his car is ready gets a smile, too. Even if he's just leaving a message.
And last, but certainly not least, you should have plenty of Marvin Gaye, Otis Redding, Ray Charles, and James Brown CDs on shuffle to bring a little peace, love, happiness, and funk to the ambiance of the shop, which will cut the stress level even more.
Good God, y'all.
