It's marketing gimic to make it look cheap

30 seconds block means 30 seconds

So 1 minute of your call is 60 seconds block so it's 20 cents/min

10 cents/minute at 30 sec/block means 5 cents per 30 sec.
which means, once connected no matter you talk for 1 sec or 30 sec, the charge is 5 cents.
in order to maximise it, always talk close to 30 sec but don't over 30 sec.

for example, you talked for 10 minutes 29 sec and you talked for 10 minutes 1 sec is the same charges.
you talked for 5 minutes 58 sec is same charges as you talked for 5 minutes 31 sec.