internetcalls.com is a VoIP provider, or ITSP (internet telephony service provider). they offer such cheap rates because they buy minutes direct from CELCs (Competitive Local Exchange Carriers), which sell VoIP minutes at very low prices. If you are used to paying 10c/min for long distance (USA) this is because you are used to dealing with traditional telcos and their rates. VoIP rates are usually cheaper because the call can usually be kept OFF the PSTN (phone network) for most of the way, and transporting across the Internet is much cheaper.
CLECs came about after the telecommunications act of 1996, originally to provide competition in local phone service. Some of that happened, however they have become quite useful due to their PSTN links, now many of the big ones sell VoIP minutes to allow ITSPs such as internetcalls, vonage, etc to connect VoIP networks to the PSTN.
As for how to get cheap calls with asterisk- you’d want an ITSP. ITSPs vary a lot in terms of reliability and support. Most ITSPs don’t charge long distance fees (USA). Check out: quantumvoice.com, viatalk.com
You can also buy minutes/phonenumbers wholesale, and pay as you use them. connect.voicepulse.com is such a provider. You pay $11/mo/phonenumber with unlimited incoming, and/or pay .5c-1.9c ($0.005-$0.019)/minute outgoing. You can also set your own outbound caller ID on such a service.
PRI is (usually) a T1- a digital circuit which has 23 channels, each carrying one voice call or 64kbit/sec of data.
However if you have *, and a good Internet connection, you’d probably want a straight ITSP. Remember that not all ITSPs work with *- carriers like vonage or packet8 don’t support BYOD (bring your own device), they make you use their phone adapter gadget which is useless with *. The providers I suggested above all support BYOD and *, and are recommended.
Hoep that helps!