well...
typically call centers have 0800 free numbers, so what would happen is thus:
The call center would have some form of digital/analogue trunks coming into your PBX (phone system) (probably a T1/E1 or several depending on call volume). These trunks would have a normal telephone number, with typically a spread of DID numbers (direct in ward dialing) ie for london let's say: 0272 111 0000- (0999) The 0000 -0999 part would be your DID range... Some of these numbers will be mapped to what are called VDM's each VDM is basically a skill set that the people in call center have for example customer support, complaints etc etc...
Ok now the 0800 free phone numbers in the USA would have destination numbers of one of these VDM's for example +44 272 111 0001 .. so your company would advertise the 0800 number as company x's customer services, and thus when a person calls this number in the USA it will end up in a cue in the call center waiting for the "next available representative".. There is a bit more to it, but that's what happens in a nut shell.
Are the call centers receiving the calls from the US through a parabolic antenna, or are these calls routed the way normal international calls are routed, which means using Panamenian-American phone companies?
The calls are routed from Telecom provider to telecom provider, in Europe at least most large telecom providers have partnerships with the other large telecom providers in each country for the very reason you are talking about.
dduardo- In my experience not may call centers use VoIP over Internet becuase the call quality cannot be controled. For internal use some companies do this, but not for external customers. If a company uses VoIP typically it will use a technology that you can implement end to end QoS so all VoIP packets can be tagged highest priortiy, ie MPLS I think BT in Europe are in a position to do this, but they own like ~90% of all the fiber in Europe
If you wanted to do what you suggested, you would need a point of presence in the source country, and a very large a costly MPLS backbone down to Panama, it would be cheaper right now, use ISDN 30 (most probably) as you get aggressive discounts on these type of lines for internation calls...
We use a Lot of VoIP, but in our call centers we only use this technology locally, ie on the LAN, all WAN voice traffic from customers is routed via ISDN E1 lines (or PSTN)
And yes Call centers are very stressful places to work