Anyone have experience with this BRI card for asterisk?

I'm experimenting with interfacing asterisk with our existing Panasonic PBX (TDA100) via a couple of BRI interfaces, and have run into some trouble getting it working.

It appears the mISDN drivers install correctly, the B410P card is recognised, but when the dialplan tries to dial(mISDN/1/${EXTEN}) asterisk always reports that there are no free channels, and the call is dropped.

"misdn show stacks" show L2 and L1 as "UP"

Any ideas where to look for help?