FM 'NAST_PROTOCOL_UPDATE' should be called two times.
1, First time SAP calls this FM always in program RSNAST00 in routine ' PROGRAMM_AUFRUFEN'
2, When you are calling this FM second time message id and message number should not be VN, 056.
I hope this is a little confusing.
SAP trying to update one information message which is not so important, since SAP kept restriction not to update NAST "if no other message exists in the remaining program".
To achieve the above scenario, SAP kept a counter for messages.