fter resetting the connectivity via systems menü
(Settings → System → Reset Options → Reset WiFi, mobile & Bluetooth)
AND rebooting the device, wifi-calling got operational.
Here is snipped, that finally got wifi calling running. Maybe it helps.
Again starting in wifi calling menu, activating the feature.
03-13 07:31:46.069 2723 2723 D NetworkController.MobileSignalController(1): onDeregistered imsReasonInfo=ImsReasonInfo :: {1000 : CODE_REGISTRATION_ERROR, 4002, null}
03-13 07:31:46.089 2723 2723 D NetworkController.MobileSignalController(1): onDeregistered imsReasonInfo=ImsReasonInfo :: {1000 : CODE_REGISTRATION_ERROR, 0, }
...
03-13 07:31:46.206 2723 2830 I NetworkController.MobileSignalController(1): isVowifiAvailable,mVoWiFiSettingEnabled = false mMMtelVowifi = false getDataNetworkType() = 18 mCurrentState.voiceCapable = false mCurrentState.imsRegistered = false, regTech = -1
03-13 07:31:46.285 2723 2723 D NetworkController.MobileSignalController(1): onDeregistered imsReasonInfo=ImsReasonInfo :: {1000 : CODE_REGISTRATION_ERROR, 4002, null}
03-13 07:31:46.287 2723 2723 D NetworkController.MobileSignalController(1): onDeregistered imsReasonInfo=ImsReasonInfo :: {1000 : CODE_REGISTRATION_ERROR, 999, General_Error17-Unable to connect}
...
03-13 07:32:29.193 2723 2723 D NetworkController.MobileSignalController(1): onRegistering imsTransportType=2
03-13 07:32:29.197 2723 2723 D NetworkController.MobileSignalController(1): onCapabilitiesStatusChanged isVoiceCapable=false isVideoCapable=false
03-13 07:32:30.350 2723 2830 I NetworkController.MobileSignalController(1): isVowifiAvailable,mVoWiFiSettingEnabled = true mMMtelVowifi = false getDataNetworkType() = 18 mCurrentState.voiceCapable = false mCurrentState.imsRegistered = false, regTech = 1
...
03-13 07:32:30.361 2723 2830 I NetworkController.MobileSignalController(1): isVowifiAvailable,mVoWiFiSettingEnabled = true mMMtelVowifi = false getDataNetworkType() = 18 mCurrentState.voiceCapable = false mCurrentState.imsRegistered = false, regTech = 1
03-13 07:32:31.165 2723 2723 D NetworkController.MobileSignalController(1): onRegistered imsTransportType=2
...
03-13 07:32:31.184 2723 2723 I NetworkController.MobileSignalController(1): isVowifiAvailable,mVoWiFiSettingEnabled = true mMMtelVowifi = false getDataNetworkType() = 18 mCurrentState.voiceCapable = false mCurrentState.imsRegistered = true, regTech = 1
...
03-13 07:32:34.470 2723 2830 I NetworkController.MobileSignalController(1): isVowifiAvailable,mVoWiFiSettingEnabled = true mMMtelVowifi = true getDataNetworkType() = 18 mCurrentState.voiceCapable = true mCurrentState.imsRegistered = true, regTech = 1
In exchange bluetooth partly stopped working 
As the reset deleted the paired devices, I was able to get a JBL Flip 4 and Anker box running again.
In case of rpi based bluetooth box (https://www.picoreplayer.org/), I’m able to finish the pairing procedure successfully and shortly start connection, it finally gets dropped silently after. It even not showing up in recent devices, so I guess even stating “active”, the connection procedure gave up at some point.
Due to fact that other working, root cause may be located in picoplayers bluetooth.