The general rule for overseas travel is that unless you desperately want a monstrously large bill, you should switch off data roaming. But Dan Warne at AusBT has discovered that may not necessarily be true when it comes to the issue of voicemail.
According to Warne, by using the data-based visual voicemail feature on Telstra and Vodafone iPhones, you can actually end up saving money over claiming voicemail the traditional way when you're overseas. A standard 30-second voicemail is equivalent to about 85KB, which equates to around $1.27 on Telstra's roaming data rate. Compared to $4.47/minute cost to retrieve voicemail from China (the example AusBT uses), and you've saved a heap of cash.
Ultimately, if you travel a lot with an iPhone 4, visual voicemail may be a better option for you, even with the monthly $5 charge.
[AusBT via Lifehacker]