should be easily done in C number tree in B number analysis. In North America we have a standardized DT template so the C number analysis tree is B=31. This tree is specified on the route for MRI (mobile routing interrogation) as parameter mis1 and activated with parm mis5. find by EXROP:DETY=MRI;
If you are talking about GSM and are using Ericsson Switch it can done creating B-Number analysis for this particular issue, you should know that BO parameter on IMSI Series analysis is indicating where C-Number analysis is going to be done (B-Number tree), so in that B-Number you should send whatever subscriber dials to an announcement, MRI route is only used if CF is going to be executed on GMSC, for roamers, who takes the call control is VLR and it is done as explained before.