Also my answer.
The 900Mhz will help you get better indoor 3G penetration. Don't expect speeds beyond 1.2MB/s even on 2100Mhz - looks like they've allocated more bandwidth to 4G. And wherever you get a 4G signal, however weak, you'll be alright with their performance.
Zong has upgraded their network, yes, but until they (and Telenor and perhaps Ufone as well) don't fix their signal interferences, their data will not be as good as Mobilink's in ~mediocre areas.
Case in point: I work in Blue Area, near the top floors of our building. Mobilink, from my seat, easily gives me 15Mbps on 4G (you know how buildings are a tough nut to crack), but Zong doesn't go beyond 6Mbps, at best. And we're talking about -92 - -97 dBm (Mobilink is weaker) signal levels - but Zong's noise levels are higher (ergo, weaker performance).
Calls: all except Telenor (can't speak for Ufone) are equal. Telenor has a lot of distortions and digital noises in their calls for some unknown reason. Since all operators tend to fall back to 2G depending on the signal (even if you're standing below the tower - I have no idea why though), Mobilink and Zong will let you continue the call on 2G without garbling much. Telenor? Forget it. But since Mobilink introduced 3G on 900Mhz, you get signals in the basements and all - which is a blessing. Zong doesn't use this frequency for 3G or 4G. Ufone uses it for 3G, and Telenor has 850Mhz... But stay away from the latter two, for different reasons.
Tl;Dr? You liar. You've read it all. First preference is Mobilink. If not, Zong.
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