That is certainly not what teh situation was when I had a BTInternet account. I moved to a Business ADSL service in 2000 and had a BTClick account which then became BTConnect.
The rule then was - it is a PAYG account and dial in every 90 days or they could attach the email address to a BT Residential ADSL account but for some reason it was not possible to link it to a Business ADSL account. I continued to use dial in - travelling a lot meant it would happen quite often anyway through until around 2006. I then dialled in maybe once every 6-12 months - access the three or four email account and that was it. But since, maybe even 2008, I have not dialled into the account - one PC does access the email every few days but that is it.
So, maybe there are customers on a legacy agreement.
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M H C
taurus excreta cerebrum vincit