Your wording makes the question a bit hard to understand (middles?) but if Optra wants to be cheaper than Teltus across the interval of (60,80) calls then you can, as you did, solve for
$$0.3(80)+15=80m+12 \Rightarrow m=0.3375$$
Looking at the graph below you can see that using a cost of $0.3375 per call keeps Optra cheaper than Teltus across the entire call range of interest. If they charge more than this they become more expensive than Teltus towards the higher end of the range.