I don't know why this is such a mystery, aside from nit-picking over the use of ambiguous terminology.
Assuming you mean the SIP proxy server, the setting is clearly available, for each of the four SPx configurations. In the basic OBiTALK web portal configuration, it's either pre-filled for the SIP service providers for which Obihai has known configurations, or you can enter it yourself. It's also further configurable in "Expert" configuration mode.
See the attached screenshots.