ADSL/CHAP Authentication process question.....

[#1]

When my (PlusNet) broadband went down some months ago, as part of the troubleshooting process (to test my hardware) I connected my router (with it's PlusNet username/password still intact) onto a neighbour's phone line - and as far as I recall (but this is basically the question...), it synched to the exchange, and logged into the PlusNet Network (I *seem* to recall seeing my PlusNet static IP address in the Router Status page - hence thinking that it actually connected to PlusNet).
But doing the reverse recently to help troubleshoot my neighbours ongoing (Homecall) broadband issue, his router will synch to the exchange when connected to my line, but won't authenticate.
So - did I recall incorrectly? Is it likely that it only actually synched to the exchange equipment, and didn't log into Plusnet?
Or, can you connect to different logical ISP's, if you simply put in different authentication details?  We are both on an exhange that is ONLY served by BT Wholesale, so both our phone lines connect back to BT ADSL Exchange equipment.... but are the exchange ends of individual customer phone lines "hard"-configured to the subscribed ISP, or is the CHAP username/password used to dynamically "switch" between, and thereby connect to, the appropriate ISP's networks at the time of logging on....?
Thanks,
P.

[#2]

Hi there,
I'm not sure if this is what you are asking, but you won't be able to use your friends username and password to connect to Homecall on your line that you have with us.  This is because your line will only be configured to use @plusdsl.net usernames.
You will be able to gain synchronisation, but you won't be able to connect unless you change the username and password to yours.  Sounds to me as though the router is working fine.

[#3]

Yep - that was basically what I was asking (in a long-winded way - sorry).
I guess I was mistaken when I connected my router to his line a few months ago - it probably only synched and didn't actually connect.
Thanks for the clarification.
Cheers,
P.

[#4]

Sorry to be a pain - but I am now very confused....
I am currently sitting at my neighbours house, physically connected to his (BT) phone line (but "Homecall" Broadband) - using my Plusnet supplied Voyager 210 with my PlusNet username/password configured into it - and it's working.
Speaking at the same time back to home (which I left connected to Plusnet via my normal DG834G), it appears that when I connect here, their Internet access stops working - so it looks like my Plusnet Static IP "moves" from my home ADSL line, to my neighbours...??
How is this possible???!!!
Cheers,
P.

[#5]

Mind if I give you a call about this?
I think if I reply in here I'll probably end up confusing myself and then you even more

[#6]

No problem - I've sent you a PM.
Cheers,
P.

[#7]

Just a quick public post to say thanks to James for yesterday's phone call and discussion/explanation as to what might be going on, even when this wasn't strictly a Plusnet problem (albeit the question raised in this forum was to do with my Plusnet ADSL/CHAP logon).
Still mulling over whether I need to be concerned about the fact that my Plusnet logon works on a phone line associated with a different ISP - and that when it's logged into Plusnet from that second location, it stops my own home Plusnet broadband connection from working (as my WAN side [static] IP address is now seen from the newer connection)..... 
I guess that there would have to be a unique situation with multiple "compromises" in place for this to be the case (e.g. someone: 1/ has discovered my Plusnet username/password; 2/ is connected to a broadband line whose ISP has not ensured that their wholesale supplier has restricted the CHAP logon to only their domain, and; 3/ has an inclination to want to use my account/disrupt my broadband service) - in which case I guess that it's unlikely to be an issue.... (so, security paranoia hopefully satisfied.... ;D).
Anyway - great CS - thanks again.
P.