We need to write two equations to start with, because there are separate charges depending on whether the time t in minutes spent on calls is less than 1200 minutes or greater than 1200 minutes. Let the cost per month be represented by C so for t<=1200 minutes, C=$29.95 for t<=1200. There's no charge till t exceeds 1200, so we can write C=29.95+0.05(t-1200) where C is in dollars. So it's a piecewise function:
C=29.95 for t<=1200; C=29.95+0.05(t-1200) for t>1200.