some of you are missing the point, I think Stephanie was being sarcastic when she said, she doesn't have money to buy a mobile phone...
Anyway, the answer is simple, delivery drivers do not call you when they arrive at your location, they give a missed call, which is difficult to do on landlines without a digital pad, that's the only explanation. Companies in Qatar are too strict with money to allow the drivers to actually make a chargeable call to the customer.
It's def not about making sure it's not prank call, it's easier to track a landline then a mobile line.