I'm on a Enterprise not Pro Hosted, and I was able to do the following. I'm sure its not intended this way but it works...
I had it self-hosted, I configured it using the custom SIPTrunk, I backed up the solution and restored it into the hosted environment - and the Custom sip trunks work. Its not an option in the drop down anymore but doesn't stop it from working. I can make changes to it ect, just as long as I don't delete it. I'm pretty sure its because it was register based SIP provider. It does show that I have a RED SIP Trunk in the dashboard, but it works fine.
I would recommend you run the Trial, set it up like a new install but don't bother with all the stuff short of the Sip Trunks. See if the trunk registers (It does have to be register base). give it a inbound and outbound call. I think you can choose Voxtelesys and then just retype the name to whatever you want to call it, change out the authentication information and it may work fine. It seems to be identical to custom register-based configuration settings.
Customizations for the Background on phones stayed on the phones too after the conversion however if I have to Factory reset them they lose the background image, I think this is because of the way YeaLink T58 Phones store background phones in their config. Even when I updated their firmwares it didn't overwrite the background only on factory.
