Early versions of DS1 only actually carried 7 bits for each sample, which was sufficient for a telephone call when companding was used to recover some of the dynamic range. The eighth bit was used for framing.
A book I have says the eighth bit was initially used to indicate on-hook/off-hook status, not framing.
This is correct. There's more. An incoming call was indicated by a solid "seizure" or activation of this bit. The receiving end would "wink" back to indicate that it would accept the call. This was ~0.5 sec blip "on" in the reverse channel. The sending channel could then transmit whatever information was arranged, e.g. internal extension number, called number, calling party number. After any data was transmitted via audio as DTMF or MF, if the receiving end decided to answer it would "seize" in return. In theory, the call ended when either end "dropped" the bit, however it was often the case that when calling into lesser modern destinations for example, the call only ended when the calling party ended it, leading to some fun empirical solutions :)
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A book I have says the eighth bit was initially used to indicate on-hook/off-hook status, not framing.
This is correct. There's more. An incoming call was indicated by a solid "seizure" or activation of this bit. The receiving end would "wink" back to indicate that it would accept the call. This was ~0.5 sec blip "on" in the reverse channel. The sending channel could then transmit whatever information was arranged, e.g. internal extension number, called number, calling party number. After any data was transmitted via audio as DTMF or MF, if the receiving end decided to answer it would "seize" in return. In theory, the call ended when either end "dropped" the bit, however it was often the case that when calling into lesser modern destinations for example, the call only ended when the calling party ended it, leading to some fun empirical solutions :)