MBD harassment

I have a similar card, bought solely for data, in my tablet. The number is not listed anywhere but MBD somehow have this number for a "handed over" client. They have been calling and sending messages for weeks which I've ignored because I don't have accounts in arrears. Eventually this became tedious so I called them from my work cellular number to inform the that I was not the person they were looking for and asked them to amend their records accordingly. Lo and behold, they latched on to my work cell number and are now harassing me on this number despite me informing several of their employees that I am not their client! Time for legal action I think!