Hi guys,
thanks a lot for your feedback! Sorry for posting back so late, but I'm currently having a hard time with the Exchange vs. Domain Controller...
Jeeez, I'm longing for the day that things keep running stable for at least 10 days in a row...
Anyway:
Yes, the adapter does have a power supply (and yes: it's plugged in

)
Yes, the 3750 does have an ISDN group.
No: I have just taken another look and I saw that
a) the adapter is programmed with an MSN
b) there is no ISDN-MSN set up for the fax extension
So I guess it will be worth a retry after configuring an MSN for the fax in the system.
I'll try that once our network shares are back up and I have some time to breathe.
Will let you know 'bout the outcome.
Cheers,
Andy