IAX2 inbound call, reachability

I have no clue what to change to fix the following problem:

I have an IAX2 configuration.
An external call goes to an Extension and then to the Follow Me setting.

If I call the extension the Telco/Carrier says the line is busy.
But if I call the same number only seconds (or sometimes two/three times) later it goes through and the caller can listen the ringing-tone or better MOH (because of “tr” as dial command option).
The IAX channels/Trunks are registered (at least I can see that they are not disrupted). N abcdef 60 Registered
xx_outbound/abcdef (S) 4569 OK (33 ms)

The IAX Registration Times are: minregexpire 60, maxregexpire 3600.

The context parameters are:

Which IAX2 setting I should change to avoid this problem?