You could do an IP-based ISDN unit, but there are three potential "gotcha's" with them at this point:
1 - They're bloody expensive.
2 - As of yet, there isn't an industry standard handshake protocol - so a Telos unit won't talk to a Musicam (for example)
3 - Since it's IP based, you're still limited to the quality of your internet connection.
You say that the SC/bridge hookup isn't as 'smooth' as straight ISDN - can you elaborate a bit? Is it the bridge induced delay that you're talking about or an excess of dropped packets on the SC side?
the bridge sounded find and would work sometimes - but as a regular thing it is clumsy
I either have to know the name of the studio so the bridge company can connect to them - which I really don't know sometimes who is actually producing the spot
or I use a company like "out of hear" and I have to call first to find out which number is available fo rthe session - then tell my agent and they call the client --- messy
I can see how the bridge would get messy like that. I actually haven't used Steve N. for anything other than a test to make sure he and I can connect and sound good, so I wasn't sure how the actual 'who calls who' works (except that Steve charges more if he has to call out...).
I have found though, that some (but not all) ISDN studios are slowly warming to SC. I think a lot of them had problems with either earlier releases or talent that had sub-par connections which made them a little gun-shy to the idea. Of course, you've also got the handful of 'golden ears' that think they can hear the bit-rate difference between what SC and ISDN can provide... :-)