You will need to talk to Telstra directly regarding any credit refund.
Boost only offer prepaid plans, and if you buy one of the phones they sell on their website, it will be seperate from prepaid plan you select.
You could also buy a phone through any retailer outright and put your Boost sim card in it. Just need to make sure whatever phone you get is not locked to another network and covers the Telstra 3G/4G network bands.
As for coverage, Boost uses the full Telstra network. Will be the same coverage wise to Telstra directly.
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