I will be traveling internationally next month and for some countries T-Mobile charges $4 or $5 per minute roaming. I would like to know if I can avoid those fees by calling via WiFi. I understand I would have to subscribe to the HotSpot at Home. Would I be able to use any open WiFi or just T-Mobile's? Thanks

Yes, you should be able to make the calls over wifi. you can do it without the H@H, too, however. Without the H@H minutes are deducted from your minute plan. With the H@H minutes are unlimited. I read that if you call back to the states using wifi there is no charge. However if you call to an internation number then you still get charged the long distance, just not the roaming.