"The number you have dialed does not exist"

I have been a Telkom client for a number of years now, I upgraded my contract more than a year ago. Since late last year when people try calling me they get the message "the number you have dialed does not exist." To date I have missed calls about job interviews, regarding repairs to my vehicle.etc. I queried this with Telkom and was told its been happening because I have a dual sim card. I have had this phone for more than a year now, how is something like this possible? Please look into this matter and have it attended to, because I cannot afford to miss out on important calls due to Telkom's network problems. If Telkom cant rectify this, I will take my business elsewhere!