DE-R-027
A Peppol Germany national rule (DE-R-*) – applies when the invoice is sent over the Peppol network to a receiver in Germany.
What this error means
DE-R-027
When both seller and buyer are located in Germany, the seller telephone number (BT-42) must be valid – at least 3 digits.
How to fix it
Check the cbc:Telephone value in the seller cac:Contact block.