The string before pound sign on a URI call to a provider is the outbound number, but should be ignored by the IP phone. You can use any character string, but I normally use "0" without the quotes. I believe some people have had problems with URI calls to CISCO phones, so maybe there is something quirky about them.
Make sure you are using the correct port number. When you omit port number it will default to 5060. I assume you are registering the IP phone to a SP trunk on the OBi. You can check the OBi System Status to see the IP address and port of the IP phone. It is listed under SPx Service Status. If you are registering more than 1 IP phone to the SP trunk, only the IP address and port of the last phone to register will be displayed.