Check if switch Q1, mostly PC4 (model: 3731) failure (see Figure 2). But the optocoupler is extremely difficult to get, if replaced with other common photo-Coupler, often calls normal, standby is also normal, but one phone call to break into, is breaking the bell rang. Main reason is that when the phone rings, the D30 is voltage at the output up to 90V, voltage Photocoupler generally cannot reach this voltage value. So, when the bell rings will make optical coupling in the photoelectric triode soft breakdown, the Q1 conduction, causing machine ring breakage, and can properly call or standby. Later in the original off-hook control circuit (see Figure 3) changes at the circuit, failure to exclude, repairing more than justify the use effect is very good. Principles are as follows: standby, because inverter 4069 1 foot high, then the 2 pin output low, QA (8050) does not turn-on relay j do not snap, Q1 is in the closing state. The phone, IC15 4 feet into low-level, PC4 photocell breakover, inverter 4069 1 foot into low-level, 2 pin output high, so QA conduction, relay closing Q1 conduction to enable the machine to get an outside line, and made the call. PC4 is available the optocoupler, Photocoupler can be used instead.