I got a call the other evening from the tenants in our rental house
saying the air was blowing but there was no heat. I checked and they
hadn't mucked up the programmable thermostat, so I figured a service
call was in order.

A technician from the HVAC company we use came out to the house for a
service call yesterday and determined the pressure switch was bad.

I don't dispute the professional's diagnostic, but I would like to
know what the pressure switch on a gas furnace does, since the office
assistant who actually called me couldn't tell me. What kind of
questions should I be asking the company?

And should I ask to talk directly with the technician who made the
service call, since I wasn't able to be there myself for the call?