@NeilR_M thanks for the follow up. Yes we have tried every possible combination we could think of to trick either system into allowing this to authenticate, but in all cases, the TP-Link SIP stack...

We run a 3CX Phone system, which has been configured for 2 digit extensions. We have rolled out a number of EAP650-Desktop's as they have a FXS port, which we intended to connect some older POTS...