The Norstar 12x0 Trunk Module (NTBB20) holds up to 3 LS/DS or Caller ID LS/DS Trunk Cards. Each trunk card adds 4 incoming lines to you Norstar System making each trunk module capable of adding up to 12 more phone lines to your existing system. The 12x0 is used to expand your Norstar Modular ICS or Norstar 824 To connect an 0x16 to the Norstar Control Unit, you will also need an 2 or 6 port expansion card if you do not already have one.