M.I.B Call to Arms is the final upgrade of Path 2 for the Monkey Village. It costs $20400 on Easy, $24000 on Medium, $25920 on Hard and $28800 on Impoppable. It is a special ability and doubles the attack speed and attack power for all towers in the radius for 10 seconds.
The MIB Call to Arms is most valued when there are plenty of towers around its own radius. Most recommended with towers that shoot large numbers of projectiles and attack fast, such as maxed TOTMGs, Aircraft Carriers, or large numbers of Spiked Mine Factories (or the other side, Spike Storms, since MIB CTA affects those abilities, too).
In BTD5 Mobile, the double attack speed stacks with each ability used, making several of these around any tower an using more than one ability can make the tower attack as fast as one shot per frame. In other words, the effect can stack.
If the player uses M.I.B. many times (maybe 10) on an Operation: Dart Storm or Ground Zero, it will look like the darts are ropes and leaving a trail of pineapples that keep on exploding (iOS only). This is now been patched.