Okay so he had to press the glass as if it was a button. But he'd touched it several times and it hadn't depressed at all. So maybe he needed more force? Or... oh! Perhaps it required Fire Bending and that was why this had not worked so far. Zuko could have said as much, but his nephew was still new to teaching. So maybe he just presumed that Iroh would have realized that he needed fire to activate it.

He took a deep breath, focused, and then pressed down with strength, pressure, and flame on his finger.

Well, his finger went down. He opened the door. "I don't think that worked, Zuko," he said holding up the melting phone.