Cannot make 2 call in the same time in yeastar S20

Hi, I have an issue now on Yeastar S20 with fw:

When i make a call from 1000 to 1001. Its ok

But when I make a call from 1002 to 1003 in the same time. I cannot make a call then 2 call do not hear any thing. Can anyone help to me to resolve this. Thank you.


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