Sync problems

Friend of mine was given an old computer, well it had usb ports so not that
old. Would it be ok for broadband was the question.

As I had recently upgraded to a modem/router I offered my old pci adsl modem
(for free) suggesting that before ordering broadband I would install the
modem on their computer and test on my adsl line first. It worked perfectly.

They ordered broadband and after confirmation that their line was activated
I returned the computer with my pci modem installed but the line refused to
sync, I also tested using the NTE test socket but still no good.

I then took my new router/modem to test on their line and worked ok.

I then checked their computer on my line again and all ok.

I`ve installed at least 8 pci broadband modems previously with no problems.

Any comments on which direction to go!!!
Do I chase BT via ISP saying they have a problem or just accept that my old
modem is incompatible on their line.
Just to add that the problem line will sync very occasionally and when it
does continues to works ok until switched off.