Whilst Shales' answer is technically correct, it could do with a bit more information.
The line port on the 110 is to connect to a PSTN or POTS service (that's a plain old ordinary telephone socket on the wall).
It allows the OBi to receive calls from the phone line and gives you the ability to route calls out through the phone line by dialling **8 before you dial the number.
This is handy if your voip service provider doesn't allow you to call certain number ranges, at least by using this facility, you can route those calls out through the phone line.
It is not for sticking another telephone into (as far as I'm aware - but I could be wrong).
