Two kinds of wiring: one is; in the home behind the telephone line, crystal head in front of the parallel connection out of the three telephone lines, each telephone line to play a good crystal head, inserted into the three telephone sets, you can meet your needs, there will be a phone call into the three telephones will appear at the same time ringing, a telephone call, the other phone can be heard when the phone picks up the phone you talk. The second is: the general telephone has two jacks, one is the entry port, one is to another phone appears mouth, the connection is, the entry phone line into the first phone, and then do another phone line, both ends of the crystal head, one end of the first step in the second jack, one end into the second phone on the first jack, the third phone is connected to the same way and the second, and the effect of its and the The effect is the same as the first effect. If a small company needs to install internal extensions, this is the need for a program-controlled switch, a lot of domestic brands, just search a lot of small switches to support one or two dozen extensions to forty or fifty extensions ranging from large demand can consider installing an enterprise-level program-controlled switches, such as Panasonic, and Siemens, there are these devices can support hundreds of extensions can be added according to your own needs. The number of extensions can be increased by adding extension boards to the programmable switches according to your own needs.