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For paid support, you should be using the Biz and Jobs forum.
Cause 28 is invalid number format, or incomplete number, but you have redacted even information about the number length. ISDN numbers often need a “type of number” and often do not include the initial access digits.
PBXtra seems to be a supported product, especially as people have dumped people onto this forum by completely abandoning Trixbox, why are you unable to obtain support from Fonality?
Thanks for the suggestion.