Check with your service provider about using your cell phone in Hawaii. Among US providers, Verizon has the most extensive network; AT&T, Cingular and Sprint get decent reception. Cell coverage may be spotty or nonexistent in rural areas, on hiking trails and at remote beaches.
International travelers need a multiband GSM phone in order to make calls in the USA. With an unlocked multiband phone, popping in a US prepaid rechargeable SIM card is usually cheaper than using your own network. SIM cards are available at any major telecommunications or electronics store. These stores also sell inexpensive prepaid phones, including some airtime.