Their addin software is likely at fault.
You will also find you may not send FAX over VOIP. You will need FOIP.
Bob
I have used windows fax console for number of years with no problems. I recently switched to VOIP and now when I send a message I get a fatal error message. The fax has been sent and has been received correctly but there is no correct end of transmison message or equivalent. I am not exactly sure what is required to give a succesful transmission sent message. Can someone direct me to the correct place to correct this problem?

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