What can cause my phones to have weak rings after a phone technician installed a dsl line and modem in my home?

I don't know if you have filters at each phone or just one in the NID outside.

Did you install a new line or did you add DSL to your existing number?

There is a connection that has a lot of contact resistance. If the splitter is in the NID, it could be bad. I'd call the technician back.