You need to send the text "READY" to 789678 from the old phone & SIM.
You should receive a text back saying the request has been successful.
The old SIM will deactivate and some time after the new SIM will become active.
(Periodically turn the new iPhone off and on to check for network).
Occasionally activation may take upto 24 hours.
If you want to say thanks > click 'Kudos'.Answered your question? > click 'Mark as Helpful Answer' also.