The T1 module actually, but I'm going to hold off until PRI is supported. I got really excited when I learned of the T1 module, but was totally dismayed that it only support 16 channels and is CT1 rather than PRI.
My question was actually prompted by my suprise that after all this time, Lucent/Avaya finall came up with a way to upgrade the system without having to replace the hardware.
Avaya/Lucent/AT&T has been building phone systems and other telecom equipment for over a century now, and they're just starting to figure out how to make the low-end systems (which are typically used by non-technical people) a little more user friendly.
The first time I layed eyes on a phone system (Legend), I was completely dumbfounded that they couldn't include at least a console port for programming (telnet would probably be asking a bit much). It was much fun programming that thing with a MLS-12 (or whatever phone I actually had at the time).
-Troy