I think an IP phone using the above technique will work for outbound calls on the remote OBi200, but I don't think inbound calls will work. You will need to point the IP phone to the Public IP address of the office and then do port forwarding of port 5063 in the office router.
If outbound calls don't work, try the following change on the OBi200:
Service Providers -> ITSP Profile D General -> X_SymmetricRTPEnable: Checked
Another technique is to have the IP phone connected only to the local OBi202 and route inbound and outbound calls over the OBiTalk network from/to the OBi200.
or
Have a hybrid setup. Outbound on a direct connect and inbound routed over the OBiTalk network. This might be the best choice if direct connect outbound works.
You can solve the dynamic DNS problem with a free account at Freedns. The OBi200 can be configured to update the IP address.
http://freedns.afraid.org/zc.php?from=L21lbnUv