«

I was wondering whether the format:

in

is enforced by the SIP standard and whether the SIP equipment
manufacturers allow a call simply sent to a host at port 5060.

In other words, is the following theoretically and/or in practical use
allowed?:

<null>@x.y.z.t:5060

If a SIP device has only one phone number, shouldn't that default
phone ring when no user is specified?

-Ramon

You can simply dial the recipients IP No. (replacing . with *).
Regards,
Martin