Really, the issue here is with MAP (an add-on to SS7 to support mobiles). The explosion of mobile means SS7 is no longer just the playing field for national carriers - mobile-only operators came to the party (still all $xbillion players). Then, smaller countries with some interesting networks came on the scene, and rather naughty SS7 traffic started to appear on the network.
Smarter operators (or at least bigger ones who got their fingers burnt) spent money to install gateways that limit and control their exposure (wouldn't you?). The less clueful/more cash-strapped/networks in less-developed countries remain more exposed.
Anyone interested can search for 'SS7 mobility management' ; the <a href="http://www.informit.com/library/content.aspx?b=Signaling_System_No_7&seqNum=116">code is easy</a>, the issue is getting access to the network.
Oh, wait, these days SS7 is being routed over IP now (ever wondered what the <a href="http://lksctp.sourceforge.net/">linux SCTP module</a> is actually for?).