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Audio issue in calls between two pbxs

Hello to all, this is my first message, my name is Alessandro and i'm an IT manager. I have three PBXs: in headquarter (say HQ) there is a MyPBX standard, and in two branches, one S20 per branch (say B1 and B2).

I have 10Mbps symmetrical hyperlan connection in each office, each one with a Linksys LRT224 router/gateway. Each PBX is connected with two others by a SIP peer voip trunk.

Codec used are: 1) G722, 2) u-law, 3) a-law.

If I make a call from HQ to B1, I hear correctly the B1 people's voice but after some seconds or minutes, people from B1 hears glitches in my voice (or broken voice or choppy voice). At the same time, if people from B1 calls me at HQ, the call is perfect.

Same thing happens in HQ-B2 calls and in B1-B2 calls. It seems that there is a random "preferential direction" because one people always hears correctly and after some time (variable) the people on the other hand starts to have troubles in hearing his partner.

Bandwidth test shows always low ping (between 10 and 20 ms) and 4 to 8 Mbps available in DL and UL.

I switched off firewalls, set QoS high priority to SIP (UDP 5060) and RTP (UDP 10000-12000) in all routers.

I saw all tutorials from Yeastar and configuration of PBXs is correct.

Please can you help me find a solution?

 

Thank you,

Alessandro

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