The access tandem is where long distance calls are switched into the local phone network for a given LATA. Local phone companies and long distance providers all

The access tandem is where long distance calls are switched into the local phone network for a given LATA. Local phone companies and long distance providers all buy into the same access tandem in order to connect into the geographically fixed market. When calls are switched across an access tandem in the US, it's the initiator of the call that is billed per minute. The receiver at the other end of the tandem switch may pay a fee to the switch operator to receive the RECORDS of which other company initiated a call in to them. That way, they know who to go after and bill for each incoming minute! Tandem operators typically don't provide this info during the call signalling, even though they could. As you can imagine, per-minute call billing is not always settled between tandem players. They can block each other based on company ID alone, and often this requires cooperation from the tandem operator as well. Some of the per-minute rates are incredibly high. The flat-rate long distance phone providers are obscuring the fact that some areas are extremely high-cost to terminate. This system has sprung into high per-minute cost billing in high-cost areas (rural LATAs) so that people operate free dial-up (in the 90s anyways), free phone conference lines, and even "free international calls" all taking advantage of the incredibly high per-minute rate by billing the maximum rate permitted in their LATA (typically not higher than the highest per-minute tariff posted by the incumbent local monopoly). This highly regulated old-school system seems so antiquated in the face of the Internet, but it survives because the tandem is the one place where all providers can interconnect, and this turns out to be the hardest part of the puzzle to work around. In the age of the internet, we could come up with something better :)