Or did you think that VoIP somehow magically works without sending or receiving data?
An uncompressed voice stream on the PSTN is 64kbit/s. Every VoIP implementation that I've seen uses compression/codecs to reduce this, sometimes to <20kbit/s. At 64kbit/s you could talk for eight hours a day and come out with a monthly total of 6.7GB. The actual usage would be substantially less than that.