Missed a call from this number, think its a virtual phone number, but when I phoned back, 5min later, the telecom message said number didn't exist.

I have received multiple calls like this over past few months, always from a virtual number, and when I have managed to speak to someone, it's always a foreign accent (Nigerian? Indian?) usually from some 'marketing company' or the sort. I usually hang up after asking them not to call again. I expect I'll be receiving more calls as they seem to have my number.