Have a customer with existing PRI from Windstream. Been working for years. In June customer noticed during busy morning times that they couldn't call out - "all channels busy" with a busy tone on phone. I put all 23 channels on a 24 button phone. Sure enough, when "all channels busy", all 23 keys lit on phone. We "governed" the two main DID down to 4 calls each (MMC714 - MC (max call). Tested = 5th call busy. Still same problem. Windstream says OS500 allowing second call in on in-use channel!!?? We have replaced Tepria, MCP2 and both PSU-B. Any help appreciated!