Just on another note there was a possibility that the US and EU DTMF schemes could have ended up being different and incompatable! The in band MF systems used for signalling between exchanges nationally and in some cases internationally in Europe were quite different to those used in North America.
Some of the large EU switch manufacturers considered using their own schemes.LM Ericsson, Siemens, ITT Europe, Phillips and the various UK companies involved in swithc manufacture didn't really have to go with the US system.
However, it made more sense to stick to one way of doing things in the end particularly when DTMF rapidly became a way for customers to control automated services.
Also the A,B,C,D tones are often used by voicemail systems to signal. E.g. when our public voicemail calls the house to notify of a call it signals D between each scentence to ensure it doesn't answer itself! If it answers a call and hears "D" it will cause it to hang up immediately and wait a pre-programmed time before it tries to notify of new messages again.
A,B,C,D also get recognised by our local switches here in Ireland.. You can dial them as part of a number and just get a wrong number announcement once 7 digits are dialled or if you just hit them as a first digit you instantly get a wrong number announcement.
The strangest bit: I accidently dialled an A instead of a 1 (old handset from an office system) after the first 3 digits and it started ringing and was answered as by someone working in an office, apparently it's part of the phone company's internal numbering for calling people in switch rooms from a linesman's phone!
E.g. 555-A will get you someone in the switch that handles 555-XXXX
