There is an international standard for the letters on the telephone dial. It is the ITU-T Recommendation E.161. The title is "Arrangement of digits, letters and symbols on telephones and other devices that can be used for gaining access to a telephone network". The current edition in force is dated February 2002. It even gives the instructions/dimensions for laying out the "star" and "pound sign" (called other things in various languages) and dimensions for the raised round "dot" on the digit 5.
You have to purchase many of the ITU publications, but E.161 is available free.
Regards.
Charles G. Gray
Senior Lecturer, Telecommunications
Oklahoma State University - Tulsa
(918) 594-8433