We have a customer with a OS7100 phone system. Ever since switching to a new phone line carrier, they have a high pitch squeal when conferencing in a third party. This system is networked with three other phone systems and there is no issues conferencing with the other locations, its only when using outside lines. The service provider has checked their equipment and they say its all ok and its a phone system issue. We have replaced two trunk cards and the uni card and still have the same problem. This issue is common on all phones in the office and all phone lines. Any ideas or any one else ever hear of this?