Thanks for visiting FixYa!
To my knowledge, there are two cellular service providers in Fiji. Both operate on the 900MHz band for cell phones. As you didn't include the name of your cellular service provider, I can't tell you whether they have roaming agreements in Fiji (If you are just visiting and have service in another country.)
If you have the Nokia E63 your phone operates on the following frequencies:
2G Network GSM 850 / 900 / 1800 / 1900
3G Network UMTS 900 / 2100
OR UMTS 850 / 1900 for the North American version of the E63.
You may be able to obtain service in Fiji by simply purchasing a Pay As You Go type SIM card and using it in your phone since your phone supports the frequencies of the local cellular providers. But, you'll need to contact one of them to find out what their sales policies are.
AND, you may need to have your phone SIM unlocked to use a SIM card other than your current service provider. (If you have a PayPal account you can usually purchase an unlock code from eBay. Just make sure you purchase an unlock code for the cellular provider you currently use.
Click for eBay unlock codes Nokia E63 > http://shop.ebay.com/ctg?_kw=E63%20unlock%20code&_ds=1&_fcid=1&_localstpos=30303&_sc=1&_sop=15&_stpos=30303&gbr=1
One other thing, you may want to contact your own cellular provider to see if they provide roaming in Fiji. If they do, they may be able to provide you service by adding it to your current account.