I tend to just to a little cafe (one where you can see another cafe / restaurant. I order something small like a coffee, and then just sit back for a while. I look for people with easy-to-see-attributes, such as a red t-shirt, a hat, etc. I then call the cafe/ restaurant and say:
"Hey, I've been waiting quite a while for my order and it still hasn't come, I've ordered a (whatever) about 20 - 30 minutes ago. I'm the man in the red t-shirt (for example)"
Sometimes this doesn't work as they would ask you to come inside etc, but 60% of the time it would. This is awesome because you can see the results. If they ever do spot you on the phone (happened to we ONCE) and they start walking across the road, quickly hang up and dail another number. Then think of a story etc.
For me this works best as a prank phone call, but using soundboards has a distinct advantage.
Good guide, but make sure to hide your number!