Grandstream UCM without sound when forwarding call to an external number
If no audio stream in using of call forward with the UCM IP-PBX from Grandstream, then the setting RTP keep-alive may be required. The required setting can be found under PBX Settings – SIP Settings – ToS.

I was litteraly hitting my head on the wall with this one. Didn’t find the information on either GS’s forum or their own documentation. Thanks again!