The delay remind you of a bad cell phone call? Delay with echo? Slightly out of freq? Your data flow is being restricted and the network is seeing errors and asking to "send again". Data doesn't care - it just runs a little slower. Voice does care. You end up with slow send - receive and your ear can hear it. Betcha it ain't the Centrex. It is nominally set to talk duplex just like any other system. The only time you would even come close to the echo, etc. of an IP connection was in the old days of long distance over carrier with a bad echo suppressor and out of balance power on the carrier. Over simplified, Centrex is just a central office using PBX features.

Ken
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