The Once-ler didn't always communicate via Whisper-ma-Phone. For example, after abruptly dismissing the Lorax, the Once-ler builds a "radio-phone" (124) to call up his "brothers and uncles and aunts" (125).
Soon, the whole Once-ler family arrives. Together, these green-gloved people chop down Truffula trees and knit Thneeds in the Once-ler's factory.
This all goes down as the Lorax, looking mighty small next to the big factory, just watches.