There is a gap in function between mobile phones and landlines.  Mobiles can send and receive text messages and landlines cannot.  This should be fixed.
Idea 1 – text messages for landlines
The network already has technology to send caller ID. The same signalling could be used to transmit text messages.  Caller ID works by sending an ascii character string to the phone between the first two rings. The signalling is at 1200 bps, and could last for three seconds, giving room for up to 450 characters.  Since texts are shorter than that, the central office could repeat the message for reliability, or we could add technology to let the phone acknowlege the signal.  The upstream ack could be modem tones or touch tones, which the phone can already generate.
Since current phones generally do not have good displays, and don’t expect text messages, this capability could be added to answering machines instead of phones.
* The answering machine could record the text for later display
* The answering machine could send a tone or modem acknowlegement
* In principle, the answering machine could have cell-phone like software to generate texts using T9 predictive keyboarding, or use a full keyboard.
* An answering machine with this capability could answer calls with a distinctive beep or tone sequence that inform the CO and equipment at the caller end that the capability to receive texts was present.
I know this is starting to sound like minitel!
Idea 2 – Texting <during> calls
I frequently call someone to get a phone number.  On other occasions, folks call me to get a number. We do this in speech.  The recipient has to write down the number or try to remember it.  When the caller is mobile, this adds to the danger of using the phone while driving.
Why not make it easy to send numbers during the call?
* If you press the keypad during the call, the phone will send touch tones.
* The receiving phone should detect this, and greatly attenuate the earphone path, so the tones don’t blast the ear, and should remember the digit sequence.  After the call, the phone could make those numbers available to dial.
* On the iPhone, and probably others, in the contact list there is a “share contact” button. This can send a contact via text.  If you try to do this during a call, the phone could send the number in-band as above.
* For landline users, an answering machine (as above) could listen in parallel to an arriving call and record such in-band messages.
Rich phone companies, send your licensing inquiries to me!