Many don’t realize that this is possible from Office Communicator (RTM or R2) – allthough I have showed it to the students at most of my boot camps.
OC according to my testing supports what is called E.161 mapping of characters to numbers (also known as ANSI T1.703-1995/1999, and ISO/IEC 9995-8:1994 or most probably similar to what you see on your mobile phone), so if you write e.g. +1800flowers in your OC client it will look like this -
The only rule is that you have to start with a number, otherwise the client will think that you are trying to lookup a person in the Address Book.
As you may know Microsoft supports RFC3966 and here is what the RFC says about this (Thx to Dennis Klama for this pointer) -
5.1.2. Alphabetic Characters Corresponding to Digits
In some countries, it is common to write phone numbers with alphabetic characters corresponding to certain numbers on the telephone keypad. The URI format does not support this notation, as the mapping from alphabetic characters to digits is not completely uniform internationally, although there are standards [E.161][T1.703] addressing this issue.
What happens in OC, is that the character/number mapping is taking place client side and therefore the actual URI only consists of numbers.
Furthermore, my testing shows that OC supports the International standard for character to number mapping, but you should be aware that different countries in the past have had different regional implementations of the mapping. Furthermore the use of vanity numbers is unusual in Europe.
For your reference I have created a table mapping of characters/numbers that OC and E.161 uses