I think the key here is the lock in safcom has on its customers in value ad-ons like mpesa,okoa jahazi et al and yeah the 20bob scratch card.
However it is also worth noting that even if calling someone on safcom can be 7.50 or even 5 bob for that matter,the reverse is 16bob which means that one maintains the safcom line. 1 Peculiar habit i have observed is if i need stuff from you,i will call you on the lowest possible tarrif (most likely you r on safcom) and let you know that u can call me on safcom coz it may cost u double to get back to me on the line i called you on & i want to remove any possible barriers that may hamper you calling back!!!