that should work. You can also make bogus SIP device. (or just switch to deviceanduser mode and make a user only, which is exactly that, at virtual user. (which, in 2.6, you will have an option to create even when in extension mode).
Ok i found the deviceanduser thanks for tip i will test tomorrow.
BTW… what is the default behavior of a SIP extension when the SIP phone is offline. if the ext is called should it go directly to VM or will it generate an error?