There are issues with pre 6.1 and PRIs. If you are happy with having to reset the system when all trunks are busied out then you could probably stay at your current level. 4.0 make a call and grab another intercom key to make another call without hitting the release key and the channel is held up until system reset. DO THE UPGRADE. I cannot stress it enough. Partial PRIs are a pain in that there is more than the usual amount of programming in phone systems to accomplish this.

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