ISDN is very important, In China too. City Shanghai alone have more than 100K ISDN user.
We had been working on ISDN for Linux for about 2 years, currently we are able to:
1. Support Teles, Winbond, etc. internal card, which all included in standard distro.(I've been working for Teles China, so it may helps)
2. Suport 1b, 2B and multi-B up to 10B bundle/5 card in one manchine
3. Support Call-back, but limited to 1B
Our problem:
1. it's not stable, kernal 2.0.38/Stable -> Kernal 2.2/UnStable that may crash the system. These days we have seperate it from the kernal
2. Multi-Channel Bundle is always not stable
3. Call-back support is stable.
Sorry, I'm not the developer in my company. If you are intersted, I may ask them to contact you.