The type of the gateway Trunk.
unknown: Unknown Trunk
other: Unidentified Trunk type
trunkGroundStart: Provides Far-End Disconnect Supervision
trunkLoopStart: Provides No Far-End Disconnect Supervision
trunkDID: Direct Inward Dial
trunkPOTS: Plain Old Telephone Service
trunkEM1: E&M Type 1
trunkEM2: E&M Type 2
trunkEM3: E&M Type 3
trunkEM4: E&M Type 4
trunkEM5: E&M Type 5
analog: Analog
pri: PRI
bri: BRI.