First item/order of business is finding an appropriate touch-tone phone.

At first this might at least appear an inconsequential matter, being seen both somewhat as a more 'obsolete' technology, and thus readily available, seemingly, at community recycling centers everywhere.

Further, with the Rasp Pi hardware alone we could consider directly 'generating' or performing playback of the dial-tones, and thus dispensing with the conventional base all together or establishing some other new or custom form factor.

However, for this build my requirements were that from the design perspective:

1. The device remain 'seamless' or that is to say 'completely' familiar (i.e. the difference is on the inside).
2. All 'conventional' functionality of the phone is preserved. Especially important if it is to be fully useful in an assistive technologies setting where, in the case of the need for emergency care (or device failure) [fire, call 911, etc] it still readily performs in the expected way.
3. Out of the box I was looking for a 'corded' solution, a wireless handset is obviously a rather needless increase in complexity. Most 'cordless' device modules already include 'audible' device location beacons, but even for those of is that can see at least alright, still sometimes it is still like 'where's the damn phone ?'

During my initial scouring of several recycling centers for a suitable device a few things became apparent. While especially in the last 10 years or so 'touch-tone' as opposed to rotary seems 'old technology' there certainly was a time when touch-tone devices represented 'the cream of the crop'.

First invented in 1887, it likely wasn't until April 22, 1963 that President Kennedy signaled the opening of that year's World's Fair in NYC via the oval office by keying in the year '1-9-6-3' that such devices truly signaled in popular consciousness the desirability of 'the future'.

Still, it wasn't until the very end of the 70's and into the mid 80's that touch tone devices commonly came to replace rotary in the middle class life style.

Why is this 'historical point' relevant to the 'modern modder' ?

Well, a couple of notable things to point out. While these phone designs such as the 'Western Electric' above now seem 'retro-cool', they also represented at least a certain level of 'cutting edge' fab technologies.

Easy to forget, but important to recall, this was the era before SMT/SMD (surface mount) devices were common or affordable options-- Wave soldering and low layer density boards for consumer devices were the norm.

Further, even today, in a fact that is perhaps less 'well known' (but makes sense if you think about it for a moment), with the exception of the latest VOIP devices, or for anyone of some age that has ever experienced a 'black out' as a child and had a 'land line phone'-- When did you ever have to 'plug it in' ? Or just how was it at times the 'phone worked' even when the power was out ?!

In comes 'Tip and Ring', a sustained standard in all un-modernized systems since at least switchboard times. Thus when the phone is 'on the hook' or 'idle on the line' a nominal ~48V is being supplied.

When a call is 'rung in' to the customer, that value generally increases 90 V @ 20 Hz.

For the true 'Hackaday' user, there are even guides on how to 'tap a line' for power in emergency (emphasis on emergency, as it is of questionable legality) situations.

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Aside from the history lesson, the relevance for this project is two fold:

1. Any touch tone say, pre-early/mid 90's (?) era is already going to be 'stuffed'. Mod in 'other ways' but don't start off imaging your going to squeeze an XYZ package in there (and certainly not a RaspPi-- As mentioned, it already has 'too much' capability in many ways for what we need here, but is low cost, easy, accessible.
2. Not all (maybe it was the 'Soviet era' I don't know ?), but many of these 'middle era' phones are not so much held together in their casing with easy to remove screws, but rather some form of adhesive. Whether it is to prevent easy 'tapping', or just the style of 'production' for the day, and of course not relevant to 'every model', many you will find kind of tend to want to break apart when you try to crack the case open.

Thus the selection of the AT&T CL2909 as my 'target device'. Full featured, held together with just a few screws on the back, and with a little thought, with ample space for whatever device. There are, however, a couple of small important points to be aware of so as to not 'break' the case, when opening it up, which I will highlight in my next post.