Is there no audio in BOTH directions? RTP ports are most likely not allowed- Need to know more about your network and Mitel deployment to determine the cause.
Might be firewall, might be MBG. If calls are establishing, then SIP port 5060/5061 are most likely open. Port ranges are documented in the Micollab engineering guidelines.
**In a typical configuration, RTP ports must be open as follows:
Internet <-> MBG<->LAN - meaning both directions.
Also, depending on firewall model/version, watch out for ALG. Maybe disable ALG, see if that resolves your issue.