PSTN Conferencing Error: Sorry, I can't seem to connect you to your meeting..."

Problem
While trying to call in to a conference from an external PSTN connection the error "Sorry, I can't seem to connect you to your meeting..."

Error Message
S4 traces on snooper revealed a "foreign gateway" IP address been called by the Mediation server.

Solution
The default Gateway in Topology Builder was an old (decommisioned) SIP connection (aka "foreign gateway". Changed that to the gateway I was actually using to call out on - solved!