I have 3 different lines ringing on my phone.

If you have lines appearing on your phone and they don't ring you need to program lines to ring. This type of programming is done from a M7310 phone. Page 323 of this manual shows how to program this on a NorthStar MICS Ver 6 phone system.
http://pdf.textfiles.com/manuals/TELECOM-F-R/Nortel%20MICS%20System%20Installer%20Guide%206.1.pdf

Enter the programming mode : Feature **266344
Password:266344
You will have 4 programming buttons on your phone, normally the 2 top left and 2 top right of the 2 vertical rows.
top left = move up one level
lower left = back
top right = select
lower right = next
once you enter the programming mode press next to "Terminals and Sets"
press select
enter your extension number
press select
press next to "Line Access"
press select
press next to "Line Assignment"
press select
use the next key to go to the line you want and use the soft-key below the display to change it from "Appear" to "Appear and Ring"

In system programming, you can select whether the display is the name, number, or line.

F**CONFIG
Terminals&Sets
enter extension #
skip Line Access, Capabilities, Name, User Preferences
Telco Features
skip Feature Assign
at 1StDisplay use CHANGE softkey to toggle to NAME.

This is tech side programming known as Configuration, clients usually call in a tech/vendor for this.
You need to find out the password and know how to navigate the system programming, do F**266344 and enter the password.
Navigate to Sets, add line in question , assign ring type.

The phone must be programmed with the target line assigned to the number you wish to have ring at that phone. This can be done with the BCM Element Manager interface. If your BCM450 is running version 6, I think this can even be done directly from the phone, but I'm not sure of that process.

If your phone system has 6.1 software or up then yes,the tech or vendor can program it for you. If you know how to program look under Config/Lines for Ring Type.

You need software version 6.1 or above rod Distinctive Ring tone for lines. programming via Config/Lines/Data

You can setup night services for each company via Service Modes.
Complex programming.

best to call a vendor/tech in for this....roughly one hour call.

To change the number of rings enter Feature 983, if your system has 2 digit intercom #s your password is 120000 if your system has 3 digit intercom #s your password is 1020000. once you have entered the password you will see 3 selections you can make .use the three soft keys under the display and select AA (auto Attendant).
The 3 soft key selections GRTG, TABLE, and LINES
Press the LINES soft key
Enter the line number on the line you want to change ringing on (probably Line 1) and press OK
Press the TABLE soft key then the RINGS soft key
Press the CHNG soft key and enter in the amount of rings you wantand press OK
Press the NEXT key to go to the next line
Repeat the procedure for the remaining lines
Press Rls when finished

Programming is done on M7310, M7324 and T7316 sets.
To set up ringing you need to enter programming by pressing the feature key followed by **266344.When prompted to enter the password enter 266344.
The screen should say TERMINALS&SETS. The 4 top line/feature keys are now used for programming.
At the terminals&sets prompt,
press the show key (top right arrow).
The display should now ask for an extension number. Enter extension number you wish to program ringing on.
Press the show key (top right arrow)
The display should sat LINE ACCESS
Press the show key (top right arrow) .
The display should say LINE ASSIGNMENT.
Press the show key (top right arrow) .
The display ask for a line number. press list
The display should say line 001: appear & ring
press the Chng key till it says appear only
press next key (lower right arrow)
The display should say line 002: appear & ring
repeat the procedure the same as line 001 for the remaining ringing lines

press the release key when finished

it may be turned down to low.press the program key and use the scroll bar below the keypad to find your option...hopes this helps

You may need to change your Ring Volume. Here are some instructions on how to do so.

To Change your Ring Type:
1. Press the Feature button, then * 6
2. Press the number keys 1 – 4 to hear the different ring types
3. Press the OK softkey when you hear your desired ring
To Change the Ring Volume:
1. Press the Feature button, then * 80
2. Press the volume key up or down
3. Press the release key1 to exit and save the volume level

Let me know if this works for you.