Oh, so you are saying that you have DID coming in on an ISDN line. You have 2 channels (trunks) and 4 DID numbers assigned to you?
In that case, the first thing you need to know is how many digits are being sent to you by your provider. Commonly you may see the last 3-digits of the number, the last 4-digits of the number, or the full 7-digits (not as often) of the number.
If the station configuration is the same as the 150 Pro I have, there are 2 boxes for the extension number, then the name, then the status, device type and port listed for each station.
The first extension box is for the number you dial to reach that phone from inside, and is the phone's "official" extension. The 2nd box is the DID number for that phone. This is where you put the digits sent by your provider for the outside number you want to ring on that phone. Each of the 4 numbers you have can only be used once - so you will have 4 extensions that can be called from outside directly (which is wierd, because the numbers typically come in blocks of 20).
Here is a line from my station configuration. My inside and outside numbers match, so you will see the same number for extension as it is for DID. It does not have to be this way, but it is less confusing to the users if you match your extension numbers to your DID block. I am receiving 4-digits from my provider, which also matches my extension numbers which are 4-digits.
6712 6712 Main Lab optiset E basic Standard SLMO24 10 - 7 Master
Hope this helps you a bit.