Hi,
I have two servers both running on BT Connect.
Both use external ISDN TA - it's far easier as they are just treated as a standard serial device, but be warned, even with external ones, some work and some don't. If you have a good read & search through this forum using my name or ISDN, you'll find loads on it.
I currently use some Dynalink TA's - they seem to be fine. If I remember correctly there are some issues over connection types (asychronous connections etc) that some TA's don't handle well. I have no experience with internal or USB TA's - you'll have to consult someone wiser than me on that issue.
Beware that BT has just changed it's contract terms and their Anytime account expressly forbids 'networked computers' using the connection. I believe that they are fed up with businesses running LANS behind a server/firewall on what should be a personal account. Their new dialer software uses 2 numbers 'in case of high demand'. They will give you one of them but the other remains 'secret'. I guess that they will occasionally pull the plug on the main number, and any one with their dialer won't really notice, but anyone without it will.......
I asked them what they were doing about, for instance, a Linux or Mac workstation, and they really had no answer. 'We are working on it' they said.
Personally I'd use another provider.............
Let me know if I can be of any further help.
B. Rgds
John