Newly launched mobile company Tru offers you numbers for multiple countries on a single prepaid SIM, potentially saving you a fortune on roaming charges and insane data charges. However, the structure of the plans is quite complex, and whether you'll save money might depend a lot on your usage habits.
Pay attention, because this gets a bit tricky. The Tru SIM itself costs $29.95 (you can buy it from the company web site, as well as duty-free on Qantas flights). That include a $15 monthly membership fee -- which you have to pay in any month where you want access to discounted rates -- and $15 worth of credit. The service uses Optus' local network, which isn't the absolute best coverage option you could have but fine provided you know it works where you need to go. Calls within Australia cost 25 cents a minute, and the same price applies to text messages. There's no expiry on credit, but if your phone has no credit you'll be charged $3 a month after 90 days has passed. You can top up credit online or through the phone.
The potentially clever bit is that as well as having an Australian number, you can also have either a US or UK number on the same SIM. (The UK network is provided by Vodafone; the US uses a number of networks.) That means that if you're in those countries, other people can ring you on a local number, rather than your Australian number -- saving them extra expenses and you roaming charges on incoming calls. For each additional number, you'll pay $8 a month.
The local rates for calls in the UK and US are pretty good too, but the big attraction is the member data rates: $0.15 per MB in the UK, and $0.35 per MB in the US. That's a lot cheaper than casual data roaming rates for your own phone, which can be ruinously expensive, and makes Tru worth considering if you want some basic data access on your phone while in the US or UK.
If you don't pay that monthly membership and just use your phone in Australia, the rates go up to what True calls 'standard' levels. Calls to local landlines and mobiles cost 50 cents a minute -- which is slightly pricey compared to most local providers, though there's no flagfall -- and data rates go up massively from 10 cents a megabyte to $2.09 a megabyte. That means if you used 7MB of data on your phone in the month, membership would work out cheaper. Weirdly, local text messages get cheaper (17 cents a minute) on the standard plan. Those rates aren't an outright rip-off, but there's ways you could potentially do better on Optus or other networks, depending on your usage habits.
Tru is aiming to expand into 20 countries by the end of the year, with Hong Kong, Spain and the Netherlands due "shortly", and New Zealand also high on the list. You'll be able to add numbers in those countries, though having multiple countries month after month would get pricey. (Tru also has postpaid plans aimed at business users, but we won't get into those right now.)
For a one-off visit to the UK in particular, I can't help thinking you might do better just buying a cheap local SIM yourself -- yes, you'll have to swap it into your phone, but the pricing structure will be more obvious, and Tru isn't so remarkably cheap that it would necessarily represent a good local deal as your single phone. For the US, it's much more appealing, given that buying a prepaid SIM is tricky and sometimes costly. Even then, I'm not sure I'd want it as my main local phone, but it's likely to be cheaper (and less hassle) than any alternative. Check out the rates on the site below, and tell us your thoughts in the comments.