thank you for your post.
The VOIP settings are designed to work with AVM FritzBox! routers which appear as an own VOIP server in your network. We don’t have any experience with your SIP, therefore, we cannot guarantee you can use the callblocker functionality as expected. Basically, the callblocker unit is added as another VOIP telephone in a FritzBox! router. Accordingly, an incoming call also rings the callblocker unit, the callblocker identifies the number, compares it to the blacklist and decides what to do. If you see any possibility to configure which devices should receive incoming calls, there is a chance it will work. But again, no guarantee.
The API key is related to the blacklist only and does not prevent the VOIP configuration from working.
Maximilian from Max2Play