Well I am replying to my own query. We found a solution.
Problem: after upgrade 4.1.2 to 5.1.2 the modem fails to connect. Existing connection is through a PABX analogue line where the modem has used 0, to get the loine and pause for dial tone. Note 0,, actually caused an echo of 000 the emergency line for Police etc. I think this may have generated the "Blacklisted" message.
Solution: With this panasonic PABX we have dropped the comma and only have 0 with the number. This works fine. Maybe someone might know why 5.1.2 needed this change. Another modem on site, used for banking programs, is still using 0, for a line with no problems.
Hope this helps anybody else who may get a similar error.
Regards
Gordon