Almost all of the problems that GMS has can be traced, either directly, or indirectly to two things. Bots, and people taking advantage of events. Bots are especially troublesome because they also take advantage of events by stocking up on mesos to sell for money. Before NexonNA can start to really tackle the problems in this game, they need a way to tackle the bot problem in this game first. Once they can stop the bots they can start to address problems such as
* maplestory2 mesos inflation
* Event exploitation
* Availability of items such as CSS, master/meister cubes, protection/safety scrolls etc...
* Devote more GM time to stopping other illegal activities such as selling mesos/nx for money
In short, if they can significantly hamper the creation of new bot accounts then they can devote much more of their time to actually working on problems that face this game. Right now, the only thing stopping bots from making a new account is getting either a new IP address or a new email (depending on the type of ban). And those are trivial to get around. However, phone verification is much harder to get around. It can be as simple as replying to a text message, or receiving a phone call and going through an automated system (eg. "Punch in your pic to verify that you are the one who made this account."), so that no one is excluded if they don't have a cellphone.
Then, NexonNA simply needs to require this on all newly created accounts. They also need to tie each phone number to a specific account. If that account receives a ban, so does the phone number. This way, once a bot account is banned, it will need a new phone number to make a new account. Phone numbers, while plentiful, are far far less plentiful than ip addresses are. It's trivial to get a new ip address, takes mere minutes, but getting a new phone number is a lot more work, and capital intensive. This places a huge barrier that only gets more effective over time (as more and more phone numbers are blocked) against bot accounts. But it does not hamper legitimate players at all (because, honestly, if you have a computer that can run MS you have access to a phone of some kind).
I know that there are some disadvantages, but I do not believe they are big enough to justify not implementing this system. The biggest disadvantage I can see is that a bot/meso farmer uses your phone number and gets it banned before you try to make your account. In that case, there could be a fairly intensive way to clear a phone number (it would have to be intensive to be an effective ban). Either that or you are out of luck, but the odds of that happening anyway are slim to none to begin with. Also, nexonNA could automatically ban phone numbers from all countries outside of the serviced countries, thus making it even harder for chinese meso farmers to work (they would need a large supply of American/canadian phone numbers and corresponding phones on hand to make more bots)