Cbw And Prepaid Sim Confusion

[Rockhead]

Hi all. What a great site! I'm hoping someone can help me clarify some of the things I'm starting to learn here and help me figure out the best choice for my situation - can you help?

I will be traveling in the Czech Rep and Germany for about 3 weeks starting next month. I need cellular service where I can be reached by callers from the US and I need to call out to US phone numbers (landlines and cell). I would like to be responsible for the charges when US callers call me.

From what I am gathering, it seems like I need a UM or Call Blue SIM AND I need a service like CallBackWorld...? The SIM so that I can have cell service and free incoming calls; CBW so that I can make less expensive calls to the US using callback and I was considering getting a direct US 800# from them for US callers' ease of reaching me.

The reason I am confused is because it seems that the UM & Call Blue service indicates that they use Call Back - so do I NOT need CBW? Should I just buy a SIM in Czech when I get there (my 1st stop) since I will be using CBW anyway? Or do I still need the Int'l roaming through UM/Call Blue, etc? If these questions don't make sense, then you see how confused I am...

The vendors I mentioned above are optional - if you have a better suggestion for my situation, please feel free to suggest...

TIA!

Rockhead