Probem with Queues and ringing phones

[Saccara]

Situation:
One incoming number will be routed to 6 phones, they all shall ring, all 6 phones are T48S
I created a Queue in the S50 and configured everything.

Now the following situation.
I take my mobile phone and make a test call to my incoming number:
All 6 phones are ringing, fine, works
I pick up the call by one of the 6 phones and I can speak.
While I speak, another call comes in, same incoming line. Now, the other 5 phones are ringing. Everything fine

I hang up my test call that I made with the first T48S. And what happens? Nothing. This phone does not ring to tell that there is a new customer. The other 5 are normal ringing

That is really a problem, becomes it happens often that only one employee is in the office, makes the first call and he can not accept the second call. He must go the telephone of the other employee.

But, I think my configuration is fine

In my queue of the S50 I added all the 6 phones, I add a Wrap-Up Time of 1second, but it does not work

Before I bought the T48S, I also had one SNOM telephone. I testet Snom and Yealink, and at the end we decided to buy the Yealink. But if I remember correctly, this feature worked out of the box with the Snom.

Therefor, can it be that I have to make a special setting in the web config of the phones? Maybe the phone must send a signal to the S50 that he is again ready for the next call?

Any idea ?