This is what I would do:

1. Go to that ext. 15 phone and press the CO buttons. If they cannot be pressed but you can see the status of the lines, the are two possibilities:

a. The phone is locked. You can try the [INT][674] test. If there is an engaged tone, then this theory is shot as the pohone is not locked.
b. The CO line access is denied, and you would have to check on Program 41. Goto Program 04 to see which port ext. 15 is, beforehand.

2. You press the CO buttons and get a dial tone but you cannot dial certain numbers:

Program 48 assigns the Barring Class to the port
Program 46 defines the Barring class

3. You press the CO buttons and get a dial tone but after dialing the any of first 4 or 6 digits, you get a double tone before being diconnected.

Program 30 LED 8, 14 assigns Forced and Verified account codes
Program 69 stores the account code digit
Program 70 Defines the Barring Class (located in Program 48) for Program 69.

Hope this helps.