Why NAT breaks calls
SIP carries the address and port for voice inside its messages (in SDP). Behind NAT the phone only knows its private address, so the PBX answers into the void: one-way audio or silence. A regular NAT keeps the same public port for every destination, so the PBX just needs to reply where the packet came from. A symmetric NAT gives each destination a new port; STUN cannot help, you need symmetric RTP on the PBX, an SBC or TURN.
How the test works
From one and the same socket the browser asks four STUN servers which address and port they see. If they all report the same address:port, the NAT does not depend on the destination. If the ports differ, it is symmetric.
What else to check
VoIP quality test — loss and jitter; SDP analyzer — which address the phone put in SDP; pcap analyzer — whether RTP arrived.