Explanation:
well,
23 < 12 + 0.04×x
11 < 0.04×x
11/0.04 < x
275 < x
x > 275 or x >= 276
so, for more than 275 monthly calls (that means 276 or higher) plan 1 is more economical.
at 275 calls is is plain even between both plans.
for fewer calls than 275 plan 2 is more economical.