I suspect a problem with your schematic. For example, the NPR transistor that drives the off hook relay would be a poor design. Most use a PNP transistor that connects to +5V - not to ground. That would also ground the opto coupler circuit since the optocoupler should not affect off-hook relay.
The typical destructive transient comes from AC mains seeking earth ground via the telephone line surge protection. A most common path to earth is through that PNP transistor, from relay coil to relay wiper, then on into phone line.
If the transient is small, PNP transistor becomes shorted causing dial tone to time out when computer power is on. If the transient is larger, PNP transistors becomes open circuited resulting in the 'No Dialtone Detected' error message.
Why a PNP transistor? Higher breakdown voltage between collector and base provides protection for the modem's computer. Your schematic provides no such protection - base to emitter.
If I read everyth> checked relay-desoldered clicks just fine but is it possible that when