With BT, everybody has the same default username and no password is required.  If you have a HH, the default username is bthomehub@btbroadband.com.  This tells the BT system that you are using a BT homehub as opposed to another router. 
If you had another router the reccommended username is broadband.user@btbroadband.com, again no password required, but I believe almost anything can be used, provided it ends with @btbroadaband.com.

Account identfication is done via your phone number.

Not sure what you have changed it to, as so far as I can recall, BT do not give a username and P/W for internet account access.

They do give one for BT BB Talk activation and for your e-mail account (the 'main' e-mail address, to which you can add sub-accounts).

Alternatively, are you referring to the BT Hub manager username and password that you use to access the hub manager pages?

That will revert to the default serial number if you reset the hub.