I believe if you're gonna do this, it must generally be the way you are taking your business model, unless you have an employee you're going to outfit this apparatus for and have him go on his merry way. If this was a weekend thing like the others were describing, you could make some easy money at trade shows, races and other functions, but you're talking about being on site for possibly days at a time. If you were 50 miles away from your shop and had two more days of installations going and you suddenly needed to be home to take care of a very important customer..... how can you be both places at once ?? More yet, I think it's very unprofessional to be working somewhere and taking business calls if you have your cell with you. If I was customer 'A' and saw you on the phone every few minutes or pacing around on my parking lot, I'd be ticked if I wasn't seeing you work on my job 100%.
When you're in your shop, it's a different thing, but on what they consider THEIR time.... your theirs completely.
We had a guy in our house a few weeks ago.... doing some repair work, I got there and he was on the cell. I let him finish and then I politely asked him to turn his phone off and concentrate on my job and let the other crap go until he was on his time. Now, you're on my time and I'm not paying you to do anything other than my work. Get done early and have yourself a blast texting and gabbing, but again, not on my time.
I fully understand someone getting a phone call about a problem or some family emergency, but not conducting business.