The ITC-100SL is a replacement/expansion wired beltpack receiver for use with the ITC-100 intercom system from DataVideo. The ITC-100 includes 4 ITC-100SL beltpacks, but can support up to 4 additional units to support 8 total users. One of the key features of the ITC-100SL is that it has a standard 3.5mm headphone jack so that you may use any headset you choose, ranging from inexpensive cellphone sets all the way up to expensive professional models. It also has a 2.5mm jack for Motorola earphones....