In order to improve fax completion rates on the SPA2100, make sure:
The SPA2100's firmware is the current version
You have enough bandwidth for uplink and downlink. For G.711 fallback, ~100Kbps is recommended. For T.38, allocate at least 50Kbps.
In addition, configure these parameters:
Set Network Jitter Buffer to Very High
Set Jitter Buffer Adjustment to Disable
Set Call Waiting to No
Set 3-Way Calling to No
Set Echo Canceller to No
Set Silence Suppression to No
Set Preferred Codec to G.711
Set Use Preferred Codec Only to Yes
Note: For T.38 fax support, enable the feature by setting Fax Passthru Method to ReINVITE under the Line tabs. Note that if a T.38 call cannot be set up, then the call should automatically revert to G711 fallback.
If you are using a Cisco media gateway, make sure it is configured with T.38 for the SPA2100 corresponding dial peer. The dial peer should include: