To withhold your number when
sending a txt is to write your
message as if you are putting a
name in the phone book but
save it without a number then
send it as a Business card.
Another way: You need a phone
with reminders on. Write what
you want and send it but
remember that the person
you've sent reminder has to have
reminders feauture in his/her phone, otherwise your number
will show up. Most of new Nokia
and Sony Ericsson phones have
reminders.