Superman asks Inspector Henderson to count into the phone handset and Superman will follow his voice to the other end of the private phone line. This is impossible because no ones voice is actually transmitted over phone lines, just electrical impulses. Those impulses then cause the speaker on the other receiver end to vibrate and reproduce the voice.
When the window from the crooks office is opened and closed there is a reflection from the bright studio lights.
When Superman arrives at the hideout [by following Inspector Henderson's voice on the phone line], he eavesdrops from outside the front door. When Lois says that involving Superman would cost her and Jimmy a raise, Superman knowingly nods, and leaves. Seconds later he knocks on the door, as Clark Kent. But he left his street clothes back at the Daily Planet building, and had no time to go back and retrieve them.