If you want to connect a single telephone line to a business system that needs multiple internal lines, consider this RS PRO RJ11 plug.
RS PRO, Male Cat6, Cat6a RJ45 Connector, Разъем
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63-4765-68 RS Pro, Male RJ45 Connector 331-6386 【AXEL GLOBAL】ASONE
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2 Pack) Ethernet to Phone Line RJ45 Female to RJ11 Male Adapter
Tech-X Telephone Phone Line Cord, Modular Jack Ends, 10Meter Rj11