No dial tone when connecting calls internally

[tvettel]

Hi,

I am using a 3CX with Yealink phones and a Gigaset N870 Dect base station.
When I answer a call on a Yealink phone and connect it to a Gigaset handset or another Yealink phone, I get no dial tone.
When I answer a call on a Gigaset handset and connect it to another handset or Yealink phone, I get dial tone.
So it seems that the Yealink phone is the cause.
Do you have any ideas?