The problem
You have set up your HorstBox (Firmware 5.0) so that you can telephone out via VOIP. However, sometimes you cannot be called. Strangely enough, incoming calls always work when you have just made a phone call to the outside.
The cause
My guess is that this is an effect of the firewall settings.
The solution
In the Internet/Firewall setting, there is an item – Block registration of SIP telephones from the external network. This item must NOT be ticked. If ticked it seems that then the HorstBox not only prevents the registration of telephones from the Internet, but also generally prevents a call connected via SIP from outside. Therefore, it is not possible to be called.
If you have made an outside call yourself shortly before, the packet filter sees it as a connection between the HorstBox and the SIP registrar, and allows incoming data via this ʻconnection’ for a short time – thus you can be called for a limited time.
Conclusion
In my opinion, this is a fault in the HorstBox firewall. This should always allow incoming SIP telegrams from the SIP registrars with whom you have set up accounts.