20-or-so years ago, there were only maybe a dozen different transmissions on the road. Every transmission repair shop had a good stock of rebuilt units on hand. There were almost no updates necessary, and even if something unusual failed, most shops had a good supply of used parts on hand to replace it.
Today there are more than ten times that many transmissions in use, with new ones showing up all the time. Many of those transmissions have several different variations or calibrations, with as many as a dozen modifications necessary.
What's more, today's transmissions are computer controlled, which means even though your transmission may not be working properly, there's a good chance the root cause of the problem doesn't have anything to do with the transmission itself.
To provide you with an accurate assessment of your transmission's condition and give you an honest estimate for repairs, technicians must perform a series of rigorous tests. They must identify which transmission is in your car, and which version of that transmission it is.
Then they have to identify the specific problem, and isolate whether it's in the transmission or the computer system.
Finally, they have to determine the likely causes for the problem, based on a logical diagnostic process. Once they have that information, the shop is able to give you a more accurate explanation of your car's condition, and put together an accurate estimate of the costs to repair it. There's just no way to do all that over the phone.