思科的技术文档中说是E＆M接口之间类型不同造成的pbx交换机。

E&M Interface Types and Wiring Arrangement

There are five different E&M interface types or models named Type I, II, III, IV, and V (Type IV is not supported on Cisco platforms). Each type has a different wiring arrangement, hence a different approach to transmit E&M supervision signaling (on-hook / off-hook signaling). The signaling side sends its on-hook/off-hook signal over the E-lead. The trunking side sends the on-hook/off-hook over the M-lead.

E&M Type I―This is the most common interface in North America.

o Type I uses two leads for supervisor signaling: E, and M.

o During inactivity, the E-lead is open and the M-lead is connected to the ground.

o The PBX (that acts as trunk circuit side) connects the M-lead to the battery in order to indicate the off-hook condition.

o The Cisco router/gateway (signaling unit) connects the E-lead to the ground in order to indicate the off-hook condition.

E&M Type II―Two signaling nodes can be connected back-to-back.

o Type II uses four leads for supervision signaling: E, M, SB, and SG.

o During inactivity both the E-lead and M-lead are open.

o The PBX (that acts as trunk circuit side) connects the M-lead to the signal battery (SB) lead connected to the battery of the signaling side in order to indicate the off-hook condition.

o The Cisco router / gateway (signaling unit) connects the E-lead to the signal ground (SG) lead connected to the ground of the trunk circuit side in order to indicate the off-hook condition.

E&M Type III―This is not commonly used in modern systems.

o Type III uses four leads for supervision signaling: E, M, SB, and SG.

o During inactivity, the E-lead is open and the M-lead is set to the ground connected to the SG lead of the signaling side.

o The PBX (that acts as trunk circuit side) disconnects the M-lead from the SG lead and connects it to the SB lead of the signaling side in order to indicate the off-hook condition.

o The Cisco router / gateway (signaling unit) connects the E-lead to the ground in order to indicate the off-hook condition.

E&M Type IV―This is not supported by Cisco routers / gateways.

E&M Type V―Type V is symmetrical and allows two signaling nodes to be connected back-to-back. This is the most common interface type used outside of North America.

o Type V uses two leads for supervisor signaling: E, and M.

o During inactivity the E-lead and M-lead are open.

o The PBX ( that acts as trunk circuit side) connects the M-lead to the ground in order to indicate the off-hook condition.

o The Cisco router / gateway (signaling unit) connects the E-lead to the ground in order to indicate off-hook condition.

从上面来看，华为和北电应该属于类型一，毕竟华为的交换机有3Com的技术成分。