freePBX answering Delay via SPA-3102

I recently installed freePBX on a Raspberry Pi and although almost everything went quite good, I’m still facing some minor problems. The main trunk for the PBX is a Linksys SPA-3102 connected to an extension of another Siemens PBX (Hicom 100E). I created an outbound route, which works flawless, for any call I’ ve made so far. I then created a generic inbound route, that forwards all calls to ring group (100).
Now, the thing is, that when I’m calling the trunk from another phone (inbound call for the FreePBX), the ring group is ringing, but if I lift up the handset immediately the call is being terminated and after 1 or 2 seconds the phone rings again for 1 second and stops. Meanwhile on the other side of the line, when the call is answered, the phone (that initiated the call) goes from the calling tone to the disconnect tone.
However, if I wait for two seconds and then pick up the incoming call, I hear a pause, while the outside phone that initiated the call still hears the calling tone. Approximately 3 seconds afterwards the call is finally established and the to sides can talk to each other.
Could someone help me, please?
Current settings
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Line details during an established call
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Thanks in advance!