1. Dont know them - let me research on them! Done - here is my verdict!
Telestial looks great, but as usual it is pointed towards the US and UK marked, but as "outsider" you are still able to get the product! The rates looks great, free incoming call in many countries, but NOT US and CA! Incoming text are free, but the data rates are horrific... somewhere between $10-30/MB is like getting robbed at full daylight...
Don't even concider that, unless there is no other option!
Hopmobile looks to me - somewhat dodgy... The rates are not very good in my eyes, and couldn't find a data rate, whitch looks to me, that if it's not possible!
Ok I hit google for an answer, and it came up with many solutions, but they are alle somewhat the same! You buy a prepaid simcard, and get somewhat good roaming charges, but it is still expencive, when you look at it overall with the startup fee! But if you travel alot to some countries, then it might be a good solution, but then again - why don't just go and get a local prepaid sim...
2. If you mean the hardware, then yes! Most usb-dongles or pc-card (of any kind) have frequencies for most of the world! The only frequency you will care about, are those for 3G networks, and for that matter, most of the world uses 3g 2100 mHz! USA for instance, have decided to use some other frequencies, and some hardware does support these too! Watch out!
3. Hmm I know that my carrier (HI3G) here in Denmark sells prepaid sim-card for data use! Just buy one of these, pop it into your modem, and your are good to go for a week or so! The price is around 150 DKK ~ $28-30, (incl 10 GB)! I don't know wheather other carriers around Europe, or the rest of the world, offers the same service, but most likely! And what regards a local SIM just for calling, those are avalible at the supermarket everywhere!
4&5. For IP-telephony I would recommend Skype! You can get one of these wifi-phones, and use it to call cheap around the world! Or if you got an Iphone, there is a skype-app for that one too, doing the same thing as the wifi-phone!
When I travel, I mostly just use my regular phone, but only for txt... When I need to make a call, I fire up my laptop and use skype...