I don't think you can do a blanket announcement,,,,
but it would be easy to add
announcement xxxx
to the beginning of your main incoming vectors for the call centers.. ect..
as for the DID's,,, you could do use incoming-call-handling treatment to manipulate the digits... and if you have variables then it would be pretty easy,, just create a vdn for each DID, and assign the real users ext as V1,, then route to same vector
1 wait time 2 secs hearing ringback
2 announcement XXXX
3 route to number V1 with cov y if unconditionally...