So, doing the math on a 15-minute phone call will give you a peak usage of 6kB/sec, 360kB/min, and 5.4MB/15min (1MB=1000kB). That’s absolute peak. 

All other usage not withstanding, that’s [very] roughly at least 46.25 hours or 2,775 minutes of talking on your 1GB basic plan.

So talk away and be silly.