Have not seen that in my SM network, I have a handful of phones (and on different providers) all showing the correct phone numbers on the clients page. The only devices not showing numbers are not actually phones but wifi-only (non-cellular) iPads or tablets. Perhaps try forcing a mass check-in, select multiple devices from the clients list, drop down the "Command" button and select Request Check-in and after a few minutes refresh device details. If you still have actual cell phones properly enrolled in the SM network and not showing their phone numbers, I'd open a Support ticket to investigate.
