S300 + yealink t21 Notification of the second incoming while talking. (Call Waiting)

We use S300 firmware and ip phones yealink t21. The problem is that call waiting is enabled, but NO notifications of the "second" incoming calls while talking are visible on the phone. I think it is the result of upgrading to the firmware How to solve the problem? What options i need to configure on the s300 and yealink t21?



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