VoIP calls are big news these days, particularly with 3's recent, awesome offer of free Skype calls for all, not just 3 customers.
One of the bigger players in the market, Vopium has just launched its mobile VoIP app for Android (yes, basically those two phones - T-Mobile G1 and HTC Magic). Like its recently launched iPhone and BlackBerry versions, Vopium for Android lets you make cheap international calls on your mobile by accessing 3G, GSM, WiFi and GPRS networks.
All new users are given 30 minutes of talk time and 30 international SMS for free.
The call quality is good, and even if it wasn't, can you complain at rates like 2p a minute to Hong Kong or the US? More rates at www.vopium.com, where you can also download the app, or just head to Android Market.