hmm...perhaps while Summoners and Callers remain unaffected, other magic classes gain access to a summon with calls that relate to their abilities.

For example, a White Mage can use Lakshmi's call "Alluring Embrace", Time Mages can use Remora's "Latch-On", Black Mages can use Valefor's "Energy Ray", etc. When a character gets a specific calling license, they gain access to the next summon up. (E.g Black Mages get access to Bahamut's "Megaflare" when they get their final license, White Mages get access to Pheonix's "Undying Spirit", etc.) The Summoning licenses would only apply to mage characters who do not naturally have the ability to call or summon.

What do you think?