I know this topic is very old but I came across this problem today and wanted to share my experience.
We have MBG as Server/Gateway and MCD with SIP Trunks via MPLS to our provider.
I was experiencing one-way audio on external calls via Teleworkers.
I found I had to add the provider's IP Address into the local networks on the MAS/MBG to create a static route via the MPLS link.
