PSTN
From aa-asterisk.org.uk wiki
The Public Switched Telecommunications Network (sometimes called Plain Old Telephone System or POTS, mainly in the United States) is the largely SS7-based network operated by the major telephone operators worldwide, to which all major telephone systems interconnect. Originally started as an all-analogue network, it is largely now digital even though the vast majority of telephones connected to the system in the UK are still analogue.
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