Not quite a calling affect, but this work for you WhamBamSam?
WhamBamSam's Conjure True Dragon
Level: Sor/Wiz 9
Components: V, S, F
Casting Time: 1 full round.
Range: Close (25 ft. + 5 ft./2 levels)
Effect: One summoned creature
Duration: 1 round/level (D)
Saving Throw: None
Spell Resistance: No
When you learn this spell you learn how to summon 8 true dragons with CR 12 or less (each configuration of feats and/or skills counts as a separate dragon; you build these dragons when you learn this spell). When you cast this spell you may summon one of these 8 dragons. By spending 8 hours a day in research for 1 week you may change one dragon you know how to summon.
Focus: 1,000 GP worth of coins.