How do ISPs know if broadband (ADSL) is available at a location ?

I ask because over a year ago, a client of mine recieved an email from IHUG/Vodafone advising them that ADSL was now available in their area.

Didnt do anything about it as they were happy with their Wired Country/IHUG connection but now their wireless router has died and apparently Vodafone are pulling the plug on wireless connections in a month or two (only notified the client when the client rung to ask for assistance...so notification failure from Vodafone) , so  I suggested they go onto DSL - I checked via the Vodafone site by entering in their address and it said yes, ADSL was available.

Great I thought, so got the client to ring Vodafone and organise a switch over - the CSR advised that DSL was not available, yet IHUG/Vodafone were the ones advertising it in their area, and pretty much the CSR told them to go to another ISP for their internet.

So who do we believe for the availability of ADSL ? How can I confirm if its available or not ?