I have compared call rates across divers plans amongs the various service providers.
No service provider offers better value for money than the Mtn Beta Talk combined with Extra Mini Pro (*142#).
The *142# is NOT a tariff plan.
It is a value plan where you can double or tripple the value of your airtime.
(Different subsribers are given different value packs at Mtn's discretion.)
In my estimation, the pack that offers #700 worth of airtime for #200 is the best pack.
Now, on my Beta Talk plan I get #500 naira bonus from a recharge of #200. Next I activate my Extra Mini Pro (*142#). The #200 in my main account is converted to #700 naira worth of airtime, giving me #1200 worth of airtime. Calls are charged at 40k/sec.

One does not realise the economy of this procedure until you compare how much worth of talk time #1000 can afford you, using the above described procedure.
[You are also given 20mb alongside the #700].
I used to be averse to any bonuses offered by network providers.
But after careful analysis, I came to know that the much-talked-about 11k/sec tariff plans can not compare to this procedure.
Not to bore you:
With Mtn Xtra Pro, calls are charged at 11k/sec, with #5 daily access fee. If you budget to use #1000 worth of airtime in a week, you will deduct #35 (for 7 days access fees). Next divide the difference (#965) by #6.6 (the value of per minute call on Mtn Xtra Pro).
Compare your answer to what you get when you divide #6000 (the value you get by using the above procedure) by #24 (cost of calls per minute).
Pls note that Mtn Xtra Pro is a tariff plan comparable to Airtel's Smart Talk or Glo's Gbam Plus; Mtn Extra Mini Pro is NOT a tariff plan, but a value pack.
Value pack lasts for 7 days with rollover of unused value appllicable.