The main reason behind the Cell global indenfication (CGI) is the MOC routing and not MTC and for billing purposes. When you get into an area where it is covered by a BTS which it’s CGI is not defined in the MSC then you will not able to make calls (but remember you are registered in the VLR), the reason you can not make A call the is routing table is not defined and this can be done only when defining the CGI at the MSC side. A good example which you can compare is the trunk number in fixed switching the purpose for it is routing and billing nothing else. Also another example is, you want the subscribers in a certain area to have access to specific number like 120, or you want some subscribers in some location to have different format of dialing, this can be done in the MOC digit checking.
As far as I know MTC is not controlled as long as it is from a legal source.
So are you working at the NSS side? If not you ask the NSS engineer what parameters he inputs when configuring the CGI? This probably includes the MOC digit table number. By the way have you experience this case that both MOC and MTC were not possible while the CGIs were not defined in the MSC? Is my explanation reasonable what I am saying just my experience. Thanks
Remember CGI= MNC+MCC+LAC+CI