[quote]Callers are the same as summoners aren't they? At least the wiki is telling me this. If you want something a little different than the description in the OP then you're free to do it.[/quote]

The Wiki is wrong. Summoners typically know Summoning spells. 
Callers know Black Magic, White Magic and Summoning. Basically, Callers are a powerful Final Fantasy IV only job class. The only way for an inferior Summoner to win against a superior Summoner is if that inferior Summoner with much weaker summoning spells also had high level black magic.

For example, Yuna could summon Bahamut, to fight Rydia. Rydia, might not have learned the Bahamut spell or other mighty summon spells yet, but she might have powerful spells that Lulu possesses in addition to her summon spells, like Fire 3, Flare, Death, maybe even the Meteo spell.