Originally Posted by soleen

Actually if there is a choice between: $25 or $45 per month it is very easy to calculate what is better: renew more often than once per month or switch to $45/1200 minutes plan.

Here is how to calculate:
PLAN A: $25 * 12 = $300 / year
PLAN B: $45 * 12 = $540 / year
Difference: $240 dollars.
This means if you renew PLAN A on sum less than $240 dollars per-year, it is better to keep $25 per month. $240 per year gives you $240/$25 = 9.6 renewals, or to keep it simple: 9 renewals.

Therefore: Total time per month you can get with $25 plan and still beat the $45 is:
(12months + 9extra renewals) * 300minutes / 12months = 525 minutes/month.

If you use less than 525 minutes per month stick with $25, if you use more, switch to $45 plan.