Adding Telephone Lines Extension Phone Socket Or Network Extension?

Phone socket or network extension? - adding telephone lines extension

I have used one in the gazebo as an office and have an Internet connection to have him as welll as the fixed network. The booth will be approximately 20 meters from the nearest point of my phone house.ie ideal to add the router at this point. So:
1) To what extent can realistically use, and manage an extension phone to use as a landline.
2) To what extent could) an Ethernet cable to run without a connection (basically between my house and my computer

I considered wireless but does not seem very reliable, with the distances you prefer to use the cabling solution, if possible.