I have a grandstream PBX, and I have one account with two registration, one softphone with one hardphone registring to the same account.
when a call is received, the two phones are ringing together, but in the SIP header, each phone has a different Call-ID.
Why is the Call-ID is different for the same call?
I don't know what a "grandstream PBX" is so I can't comment on it's behaviour.
If your SIP Proxy supports forking then the call-id should not change.
My guess is that "grandstream PBX" is not a SIP Proxy Server but is instead acting like a B2BUA, this would explain the multiple call-id's. One for the inbound call and one for each out-bound call to the endpoint devices. I would guess it would do this for media traffic reasons. That or the grandstream pbx is not following the SIP RFC correctly for forking sip proxy rules.
Without seeing network traces to confirm or reading/understanding the grandstream pbx documentation, it's hard to say.
SIP can be pretty flexible and is bent to work in lots of weird ways to make incompatible H/W and S/W implementations talk to each other without problems. This could be one situation implemented by the grandstream pbx to make devices talk to each other with as little problems as possible.