I have been able to do this in the past, but I've lost the ability to change a user's IP phone number. When you go to Phones & Devices>User Phones>Click On User>Edit Number, the only option that presents itself is to buy a new number. It no longer gives me the option to choose an existing assigned number. I had to call into Ringcentral support the other day to accomplish this and they stated that it appeared to be a glitch but they haven't confirmed it's company wide. This has been going on for weeks and I would love the ability to do this!