In my many years in the answering service and paging business and even more years in ham radio I've seen it all. Dialing by sound is possible but not terribly reliable. When there were still a lot of dial phones around there were hand held boxes that played tones into the phone mouthpiece to allow you to check your voicemail or send digits to a pager. The ones that worked the best had nice sized speakers and a rubber seal. Most of the time the problem was twist. The combination of the speaker, the mouthpiece and the crummy phone line would make the two tones arrive at the destination at unequal amplitude. Today's DSP doesn't care much, but the toroid cores and finicky AGC of those days couldn't handle much more than a few DB difference between the tones.
- Ron
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