The Farscape Wiki says:
"Comms or communication devices were integrated systems connected to and controlled by Leviathans such as Moya.
The devices had a fairly wide range, but a distinct limit. Portable units could be attached to clothing and used to communicate with other portable devices in addition to the Leviathan and her Pilot."
-- but it doesn't say what the limit is or have any references to their justification of that. I was sure that they were FTL communication but can't recall why.