This can be done, the tricky part is how familiar are you with IPS programming?
Allow digit conversion for your PRI trunk route CMD 3518>rt>0
Pick an unused DID number from your DID range and assign it to a number conversion block. CMD 7600>did>000 to 999 (pick one).
For day mode use CMD 7601>same number conversion block from CMD 7600>D16 (D16=DISA) For night mode use CMD 7602 and assign the same conversion block to D16. You then need to set up a code so you do not get hacked.
Use the following CMDs:
2AA0>2>09
2A09>Code (up to 16 digits)>0000-2999 = pattern number
2A10>000-2999>2
08>216>0
08>217>0
42>13>01-16 = number of digits in code. DO THIS BEFORE 2A09.
When you call the DID you will get a tone, dial the code, then an extension or 9 + phone number to get outside the system.