We changed phone provider, the phone line was connected but no broadband. I spent the usual lifetime with tech support. They could see that my setup was using the correct username and password, and they claimed it was "connected". We had to try another modem (Netgear wireless) and also factory reset, no good. They were within moments of raising a request to get a wiring team to investigate, which could well have cost us money.
Then almost on a whim, I asked about the other ADSL settings. I could see that two of these had somehow been set to empty values, rather than sensible defaults. The tech support woman agreed and told me the good values. It all worked immediately and avoided days of hassle. So, at least for my modem, you need the VPI to be zero, and VCI to be 100. No I have no clue what this means.
In case you don't know: typically (not universally) you can control a home broadband modem by starting a web page and visiting http://192.168.0.1. This is a page on your modem, not on the internet. You''ll be asked to login; again typically this is admin and either admin or password. Actual mileage may vary.