Several things you will need to set up.
Set a 4-digit system password in #403
Set up Disallow List 1, Entry 01 with the Wild charcter
Set up Allow List 1 through 6, entries 01 through 10 with the local prefixes
Set up Allow List 7, entries 01 through 10 with the toll free 1 and area codes
Set up Allow List 8, entry 01, with the Taxi Cab number (all 7 digits)
Set up the Emergency List with 911
For Ext. 14 - assign the Disallow List, and Allow Lists 1 through 7. This will allow all local calls, toll free calls, and for long distance: Before lifting the handset, press Hold, dial the 4-digit system password, lift the handset and get the outside line (through the pool button) and dial the call.
For Ext. 15-19 - Assign the Disallow List and Allow lists 1 through 6. This will allow all local calls, and 911 (it is in the Emergency list, and I never recommend blocking it to anyone)
For Ext. 20-22 & 27 - Assign the Disallow list, but no allow lists. This will let them get dial tone, but if they dial anything other than 911, it will cut them off.
For Ext. 23-26 - Assign the Disallow list and Allow List 8. This will allow them to get dial tone, but if the dial anything other than 911 or the taxi company, it will cut them off.
I would still go ahead and use the pool function that we talked about above.
Hope that helps!