This would be illegal to do over here but is technically possible atleast on our IPKII's.

Emergency numbers by default are in the common permit table. Any number here will override all other barrings in other tables.
All phones by default are following toll restriction class 2, which has the common permit table defined. You need to remove the emergency number from the common permit then add the number to a deny table that applies to the toll restrict class you have, blah blah.
Note this may not be correct for USA firmware.

21-04-01 (ext toll class) (default: all extensions all modes class 2)
21-05-01 (toll class to table) (note class 2 above)
21-06-04 (common permit table)