How do I text one of my contacts telephone no's to another contact, without writing it down and then re-entering the number in a new text

you can go to your address book. hit menu > select > select the number you want > menu > copy > go to the SMS message > menu > paste

Wirelessly posted (8330 Sprint)

Doing the copy and paste is easier if you swap back to the message using alt-escape.

you dont have to hit alt-esc. when in an sms you could just hit the end button and it will still keep the sms open in the background as well

wouldn't you hit the back button, not the end button?