"Being called in by a unit" implies good radio communication between infantry and aviation elements. This kind of communication was unreliable in WWI. In 1917 Britain introduced designated ground attack aircraft. At the Battle of Cambrai aircraft were integrated into the attack plan to support infantry by strafing enemy trenches and machine gun positions. Air support of infantry at Cambrai was judged to be effective. Germany's experience with the Condor Legion in the Spanish Civil War caused them to develop effective ground to air radio. Germany's use of close air support at the start of WW2 was highly effective. Good source is Jonathan House, Combined Arms Warfare in the 20th Century.