No Audio on outgoing calls only

I’m using FreePBX only for forwarding everything from my Telekom SIP-Trunk to my Skype for Business Server and vice-versa.

I have two trunks, one for the telekom connection, one for SfB.
I have two Outbound Routes, One Incoming Route.
Incoming Route routes everything to SfB trunk.
Outbound Route for SfB (first rule) routes dial pattern +493012345XX to SfB
Outbound Route for Telekom (second rule) routes everything to Telekom Trunk

So far incoming and outgoing calls are working fine, except the following:
On incoming calls everything is working fine, on outgoing calls both parties cannot hear each other.
In “rtp set debug on” there are only RTP packets from the SfB displayed, nothing from the ISP
In incoming both IPs show up (as the audio is working).

Could this be a NAT issue, even though incoming is working? Im only using PJSIP and I’ve set up the external IP in the NAT config.