If the SOAPAction is missing or defined to an emty value into the WSDL of an external service provider, the Petals BC SOAP generates a SOAPAction set to a value as 'urn:<operation>', instead of an empty value. And so, the request is rejected by the external web-service because the SOAPAction has not the expected value.
It should be better if output SOAPAction stay same than in WSDL.