How to properly link 2 phone cables with different diameter?
Typically the line from your provider is, or should be, terminated in a "customer interface" or "network interface device" which also usually have surge-suppression for the telephone line built-in - and those have large threaded studs or other screw connectors for the larger wires to connect to.





I frankly find it odd that this would be missing, as it's remarkably standardized across many countries; and the surge-supression is not something you should be without, since telephone lines can pick up all sorts of unpleasant things in thunderstorms.