They probably knew exactly how to make a phone call, but the company may not have provided them with a means to do so.
And no, "Just use your personal phone" isn't an answer. I keep my personal stuff and my work stuff strictly separate from each other. If my company wants me to do something business-related, they provide me with all of the equipment and services necessary for me to do it, or it doesn't get done, plain and simple. When I'm out of the office after hours or on vacation, I'm not working. So no way in hell some random client is going to get my personal contact information via caller ID.