Change the system
If it's really that simple to scam, there's a problem with the way you pay for those things.
I.e. if Scams'R'Us can make transport that data, in-near-realtime, between an Haitian number and the UK for less than other operators, then you have a problem.
There are three stages - Caller to some centre of operations, centre of operations to remote country, remote country to recipient.
If Scams'R'Us can perform the middle end of the connection for free or for very-much-reduced cost then you have a BIG problem in charging so much for that part. Because any idiot could stroll up with a local broadband connection and pipe VoIP lines transatlantically and beat you at your own game by buying a T-Mobile phone and a computer at both ends, say. They're still paying for the phone calls at both ends, still paying for their Internet connection (which, in turn, must pay for the transatlantic connection to be profitable), and still providing the same service that you are.
If the call quality is not bad enough that people are complaining, there's NO reason why T-Mobile can't do the same - this is how Skype's entire business plan *works* and makes them money.
Either: increase the costs of the middle part artificially and throw out the scammers (which is almost impossible but apparently what the telco's want to do), or lower your termination costs to foreign countries using the same transports and techniques that the scammers were using. If I can send hundreds of Gigabytes of data to Australia in fractions of a second from a bog-standard home ISP line, then why would telco's have problems doing the same given the amount of infrastructure they own? Sure, there might be a tax or something in the middle but the problem stems from artificially-high termination fees for international calls, or so it seems.
It sounds like a synthetic economy to me, where people make money on the back of porting packets from one end of a cable to another and then charging FAR more than it costs to do so, and when someone else comes along and does the same a different way, without people or phone companies being able to even notice, for much, much less they all start shouting.