INTV System III with a phone jack

[INTV System III with a phone jack]

Ok opened the case and it looks like there are three wires from the phone jack:

 

 

They are soldered here

 

[INTV System III with a phone jack]

Here is a picture of the phone jack

 

And an image with the phone jack cover removed

[INTV System III with a phone jack]

I just purchased an INTV System III and collection of games on eBay and upon opening the package I noticed the system has a phone jack on the left side. Any idea what this is for? I can't find a reference anywhere including in the manual that came with the system.