Android Question SendDtmf (10) does not send any tone

[V.Wolpers]

Hello everyone,

I have a problem with SendDtmf(10)

The existing devices and their IP (manufacturer/type):
PBX 192.168.53.10 (Auerswald/Commander4000)
2 IP telephones 192.168.53.70 .71 (Auerswald/Confortel1400ip)
Door station with 2 call buttons and camera 192.168.53.53 (Lippert/intelliport)
Samsung XCover3 and Google Pixel2 with my app.

When someone rings the doorbell, the PBX initiates a group call to the telephones.
My program then shows the camera image and establishes the voice connection to the door with the green telephone symbol.
The door opener can be triggered on normal telephones with DTMF *.

Private Sub ivTueroeffner_Click
    If CurrentCall.IsInCall Then
        CurrentCall.SendDtmf(10)
        lblInfo.Text = "Sende DTMF * zum Tür öffnen"
    End If
End Sub

But I don't get a beep with the CurrentCall.SendDtmf(10) command. What's the mistake?

 

[V.Wolpers]

? Does no one have an explanation?