[dr_lew]

I've always wondered about the technical details of the famous Hotline ringtone, which became sort of a character in its own right. It seems to involve several interleaved electronic tones which have a fairly raw edge to them. I don't think it was done with a synthesizer, per se, as it was early days for that, but I do think it was electronic.

[Yo-Yo]

I agree that the hotline tones are iconic. I would assume that rudimentary tone generators existed back then, since the Touch Tone telephone dialing system was designed in the same era.

[Steely Tom]

The OMF Hotline Ringtone may have been inspired by the DTMF tones that were being rolled out by AT&T in the second half of the 1960s.

However, it is not comprised of actual DTMF tones. This is clear because one of the two tones in the dialing sequence plays out a simple musical melody; actual DTMF tones are were chosen to be completely unrelated harmonically and so cannot be used musically.