We have 2 124i's which have trunks lock up about once a week. Sometimes you will find a phone with a green LED lit for the locked up trunk, but, you are unable to access the line at all from that phone (not always the same phone or the same trunk). We can force release the trunk and everything works fine.

Have you tried rolling the trunk cards to see if the problem follows the card or stays with the line?

Thanks for your response. The problem occurs on several different trunks. Since I posted this I've talked to NEC and they think it may be a software issue, so we're going to update that and see what happens.