Shutdown the symposium, then go in ld 48 and do a DACR xx where the xx is the ElanID defined for symposium. Then go in ld80 and do a trace on the TN of the voice port and see if MSB is activated. If it is, nul the key and insert it again on both TN's. Make sure ALOG is set to yes. Then restart symposium.
I'm not even close to be sure if this helps you out but this was the trouble I had and it helped me.
One more thing. Since it worked before, did you install any patches on the PABX prior to the problem...?
i2007