How do I manually rewire a phone jack to use the main line number?

I am about to disconnect a second residential telephone line in my house. To avoid a technician fee, I want to attempt to just re-route these phone jacks to use the main line number.

I have opened up the wall jack and compared it with the main line… they are exactly wired the same—4 wires (black, red, green, yellow).

Is there a simple way to do this? Credible how-to web links are appreciated!