I have no hands-on experience with OBi50x devices, so anything I write about them is based solely on reading OBi documentation. I think that most of the frequent posters here are in the same position. It would be beneficial if anyone who can post with more authority on this topic would do so.
My understanding is that only the OBi202 is able to be configured as a SIP server for IP phones to register to. I have looked at the 508 Admin Guide and do not find the X_Proxy option which should be set to 'yes' as per page 4 of the workbook. It is present in a 202. As such, how much of the functionality described in the workbook can be obtained? Have you tried it?