So here's my take on it........

The Magix provides "dial tone" to the 1A2 KSU - LInes 1, 2, and 3 are 3 separate extensions, that each have their auto line select set to go off hook on a specific outside line.

Such as Magix Ext. 31 is connected to 1A2 CO 1, and when you go off hook on a 1A2 phone on LIne 1 you are getting outside dial tone of the actual outside line through the Magix

Magix Ext. 32 auto line selects Line 2, and is connected to 1A2 CO 2, Magix Ext. 33 auto line selects Line 3 and is connected to 1A2 CO 3

Now for CO 4......
Lets use Magix Ext. 51, connect it to CO-T&R of a separate 400 line card. Jumper that to 4-T/R, A/A1, LG/L of a single 1A2 set. Strap the B1/R1 for buzzer ringing, pick a spare pair in the mounting cable, and install an additional buzzer in the phone on that pair. You would still have the bell ringer on Yel/Slt, and the buzzer for the 1A2 intercom on Yel/Org

Now connect Magix Ext. 52 to a different 400 line card, jumper that to the Line 4 position of a different 1A2 phone, strap for buzzer, install in phone, etc.
Do the same for a 3rd phone, using Magix Ext. 53, a separate line card, yadda yadda

When you pick up Line 4 on any 1A2 phone, you are getting internal dial tone from the Magix, and you can dial and of the 5x extension numbers to ring through to another 1A2 phone. Instead of ringing, it will buzz with ringing cadence on the additional buzzer that you have installed inside of the phone (because rigging up a diode matrix to hit the bell ringer would make my head explode)

Picking up on Line 5 of the 1A2 phone is traditional 1A2 intercom, such as a Melco MC-9, which will buzz through to the regular buzzer on Yel/Org

Picking up Line 1, 2, or 3 will give you outside dial tone, although through the Magix (and I guess the question here is WHY, unless you want to also have dial tone available to Magix multiline phones)