Name the stage that failed
Record the sending and receiving equipment, direction, page count, time and displayed error. Check whether the call reached the destination, whether fax negotiation began, whether any pages transferred and whether both sides reported completion. Repeat with a known test document and a known destination. A failure with one remote machine suggests a different investigation from failures with every destination. Avoid changing codec, speed and error correction together before retaining a baseline.
Inspect the actual media mode
Capture authorized signaling around fax detection and any re-INVITE. Determine whether T.38 was offered and accepted, rejected with fallback, or never attempted. For G.711 passthrough, confirm that the full path stays compatible and does not introduce an unexpected codec conversion. Compare RTP or relay-path loss and timing with the failure interval. A successful voice call does not exclude impairments that disrupt fax. Also inspect local analog cabling, ring behavior and ATA regional settings before attributing every failed page to the provider.
Validate the recovery
After one bounded change, test both sending and receiving, short and multi-page documents, and the remote destinations relevant to the business. Record completion at both ends where possible. A sender's apparent success and an unreadable received document are not equivalent to successful delivery. Retain only necessary diagnostic evidence. Fax content can be sensitive; use a non-sensitive test page and redact identifiers before sharing traces. If the supported path remains unreliable, escalate with exact timestamps, negotiation results and device versions instead of repeatedly resetting the ATA.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.