As a transferring controller, if I was giving a ROC I wouldn't transfer the a/c until clean of any confliction. Even if I was transferring an a/c without any release restriction I wouldn't transfer it until clean. If there was conflicting traffic then a release subject radar could be appropriate. As an accepting controller and the pilot called me on box 2 I would know it was not released. I should also be able to see any conflicts or ring the transferring controller if in doubt. As a pilot I would always say that I was calling on box 2 as we could be calling from well outside the sector coverage. Also on box 2 you don't transmit all the initial spiel that you would if you were on box 1 on initial contact.
I don't think ROC is banned, I think it is a fallback from the early days. Check out MATS Pt1 CAP 493, it might say in there.