Callkit - No audio if starting a call from background

This started to happen since iOS 13.3.1

On my app I use Callkit + WebRTC to establish VOIP connections. I always managed to establish connections without an issue.

However, since 13.3.1 that I'm not able to start a Callkit call if the app's not on the foreground: I manage to establish the connection but the callkit isn't started (no green icon/bar on the top) and the microphone isn't picked up also.

I always get the following error:

Error requesting transaction ((
    " contactIdentifier=(null) video=0 relay=0 upgrade=0 retry=0 emergency=0 isVoicemail=0 ttyType=0 localLandscapeAspectRatio={0, 0} localPortraitAspectRatio={0, 0} dateStarted=(null) localSenderIdentityUUID=(null) shouldSuppressInCallUI=0>"
)): (Error Domain=com.apple.CallKit.error.requesttransaction Code=6 "(null)")

From what I've gathered (there is almost no information about this code 6 error) Callkit may terminate if the AudioSession isn't active. However I'm not understanding what happened since 13.3.1 to affect this on background (I have Audio,Airplay and PIP / Voice over IP / Bacgkround fetch) modes active.

In the meanwhile I tried inumerous things, from activating the session myself (both before callController.request and also before provider.reportOutgoingCall)

do {
    try AVAudioSession.sharedInstance().setCategory(AVAudioSession.Category.playAndRecord, mode: AVAudioSession.Mode.voiceChat, options: .mixWithOthers)
    try AVAudioSession.sharedInstance().overrideOutputAudioPort(.speaker)
    try AVAudioSession.sharedInstance().setActive(true)
} catch {
    print(error)
}

to playing a silent audio (trying to force the AVAudioSession to activate) but had no luck whatsoever.

Any suggestions?