User blog comment:Omega natsu2/Fairy Tail Episode 208: Astral Spirytus, Review./@comment-2214433-20141115081905/@comment-7418318-20141115150227

A Mage has to be able to call a Spirit in order to sign a contract, so there is more to it. At least you have to know the correct incantation to call a Spirit for the first time, as was seen when Lucy used Plue's, Virgo's and Sagitarius's keys for the first time.

While it's not truly canon, one of the Omakes showed someone mess up with the summoning process and ended up calling a malformed version of Plue.