I've noticed that the length of time you set the message to display for is a bit wonky - it defaults to 5 seconds I think, but it lasts maybe a second and a half and disappears. I actually set it for 12 seconds and then timed it (by calling from my cell phone) - the actual length it stays is somewhere between 6-9 seconds, and it differs with every call.
I just got off the phone with the MTS Test Centre. The technician confirmed that in some cases the call display doesn't pop up on the TV even though it appears on the phone. He said that he had the same problem at his home and he is on a different switch than I am. He also said they had gotten problem reports from various parts of the city.
I don't know if some lines are reliable and some aren't or if it is completely random. My own testing with my cell phone and watching various calls, about 25% of my calls don't display on the TV. Interestingly, one of the calls that didn't display on the TV but showed up on the phone was a callback from MTS Repair.
Calls that don't display on the TV also don't appear in the Message Log.
We do not subscribe to call waiting. I have seen the TV display a phone number on the screen when the line is in use. Like other people have said I have noticed that call display does not work 100% of the time. I have noticed a few times where the number did not show up.