Jack is RJ-11(6P4C) or RJ-12(6P6C). The RJ11 jack has 4 conductors, RJ12 jack has 6 conductors. There's plenty of room to work. RJ11 or RJ12 Surface Mount Box. Connector A : RJ11, RJ12. What will you receive : 1 x RJ11 or RJ12 Surface Mount Box.
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