go into programming
find the extension you want to ring under terminals and sets.
go to T&S/ line access
If the lines already appear at said phone, change them from appear only, to appear and ring.
If you cannot get it to work how you want from my above description, I would recommend that you call a vendor. Please be advised that changing ringing lines can also effect how your voice mail works (if you have any,) which would be one more reason to call your vendor.
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