You're right - though it's more about the precise timing of the tones than the frequencies themselves. The T.38 protocol exists to allow faxes to send over a p

You're right - though it's more about the precise timing of the tones than the frequencies themselves. The T.38 protocol exists to allow faxes to send over a packetized network. Already if you're sending a fax there's a high chance it's been converted to T.38 at some point, as SPs gradually move toward a fully packetized core.