well, there is a router and an ISP (fixed IP).
the router has the relevant port forwarded to the SNOM telephone. Besides, it is used with another VOIP provider and everything works just fine.
as to PBX, my telephone registers with the PBX with the line parameter. They can also send the OPTIONS command with the line parameter. However, the INVITE message always comes without the line parameter for some reason
what is the line parameter used for?