If the number you want to keep is with another network then you can request a pac code then then port it to three.
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If you signed a new contract, with 3, rather than an upgrade, you get a new number. Try calling and see if you can get a pac code for your old number. If you can then you can port the number to another network, get a PAYG sim from the new network. Once your number has been transferred to the new network ask them for a pac code and transfer the number to your new 3 contract.
There are a few posts from people who have had to do this.
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