Dry refers to the lack of DC current or ring voltage found on regular phone lines. If you can get access to this audio channel, you can use the telephone base to select, dial, and put calls on hold. This gives you multi-line call handling with audio on one pair. read more
When a phone goes off hook, the voltage drops considerably to perhaps 6V or so because of the voltage drop in the line card relay and the resistance of the line. Because telephone-type twisted pair cabling is now universally used for Ethernet computer networking, 48V was the logical choice of the IEEE 802.3af Power-over-Ethernet (PoE) standard. read more
The standard voltage for the telephone system is 48 volts DC. Ringing voltage is much higher, around 90 volts AC at a low frequency. When you place a load of about 600 ohms on the line, you are "off hook" or connected. read more
The POTS phone line, with all phones on-hook, should measure around 48 volts DC. Taking a phone off-hook creates a DC signal path across the pair, which is detected as loop current back at the central office. This drops the voltage measured at the phone down to about 3 to 9 volts. read more