Thunder Call (Su) is a performance, and normally this prevents you from starting with inspire courage active and activating thunder call without dropping inspire courage:
A bard cannot have more than one bardic performance in effect at one time.
At 7th level, though, you can start inspire courage as a move action, so you could have inspire courage up almost all the time ("almost" because there's that brief moment you're using thunder call). This would eat two rounds of bardic performance, though--one for the thunder call and one for inspire courage.
There are a couple of other ways to do it, such as virtuoso performance (which costs yet another round of performance) and shadowbard (better but pretty high level). By that point, though, you can do the 7th-level thing.
(Un-?)fortunately, the design team thought of that one this time:
Thunder Call (Su): At 3rd level, the thundercaller can start a performance to unleash a deafening peal of thunder. This allows the thundercaller to spend a round of performance to create an effect similar to the spell sound burst (having the same range and area and allowing the same saving throw). At 7th level, the sonic damage dealt by this blast of sound increases to 3d8. This damage further increases by 2d8 at 11th level, and every 4 levels thereafter. A creature that succeeds at its saving throw against thunder call is immune to the stunning effect but still takes sonic damage. Beginning this performance is always a standard action.
So while it gets stronger at 7th, you still have to expend a standard action.