Is the "username" required to place a SIP call?

I was wondering whether the format:

snipped-for-privacy@host.domain

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?:

@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