Lots of people don't have work cell phones. Even if I did, and I gave you (or any provider) that cell number what would that prove? How could you verify that that number belongs to the company I work for, since the prefixes may be different? Also, even if I did, much of the building doesn't have good cell phone reception, so I couldn't get the call anyways.
I don't know why they can't leave a message with some key phrase in it, like "your car is ready" or something, and then repeat that when I contact them since then they know it's me.