Have you tried this...An apple forum say...Just hit the number associated with the letter as shown on the keypad.
The individual keys can be any of those 3 or 4 letters, so 1 press of "2" corresponds to either an "A" "B" or "C". So dialing 1-800-555-APPL would be 1-800-555-2775 (just an example).
You can't enter letters and I'm not sure why you'd want to in this instance, it looks like you're trying to use the GSM codes to divert all calls (voice, data, fax etc.) to vms02 after being unanswered for 25 seconds. What are you trying to achieve and which network are you on?