Most generic sip providers only require a proxy server, user id and password. (Everything else left at defaults)
A nat proxy is not always required, I always recommend not adding one unless It's needed. You should always confirm what the nat proxy address is as some providers use the same address as the proxy server and others will have a dedicated nat address. e.g nat.xxxxxx.net
What does the log show? Is there an error code? Have you double checked to make sure your user id and password is correct?
If you cannot get your device to register with just a sip proxy address, user id and password then try adding the outbound proxy (nat address) but don't forget to save and reboot. If your device still will not register the next step would be to add the URI under credentials in the following format: firstname.lastname@example.org
and once again save and reboot.
If your still not having any luck try adding the sip.xxxxxx.net proxy to the RegistrarServer field and UserAgentDomain field. (Can be found under Obi expert)
A little bit of advice... I always find out what the IP address is of the proxy server and enter the IP address in the proxy server field rather than any address such as sip.xxxxxx.net, prevents any potential DNS issues later down the road.
The Sipura configuration in your original post on an Obi would be:
ITSP Profie X > SIP >
Voice Services > SPx Service >
X_NoRegNoCall: No Tick
Voice Services > SIP Credentials >
AuthPassword: YOUR PASSWORD
G711U codec is set as the default codec with the Obi202.