Timescale Related Charges (TRC) ..... or 'The fee' as most would say, is largely dependant on the type of task built by the ISP.
There are too many scenario's to go into, but I'm certain the angle your coming from is the elusive intermittent fault ?? As I have said before, it all hinges on the conversation between the EU and the CP/ISP as to what task is raised (PSTN or DSL).
For the very most part, on PSTN faults the TRC is automatically built into the task when the engineer completes his task on his Laptop/i Phone via drop-down menu's.
On DSL tasks, the ISP will have already paid OR up-front for the visit ..... requesting which 'modules' they want the engineer to investigate. The more modules, the greater the cost.
We have a team of TRC 'Spotters' who are similar to konrado investigating attenuation/SNR values .......... in other words, no stone is left unturned and full scrutinisation is applied when they contact the engineer asking why a TRC was not raised, should that be the case ?
"e.g. what happens if the engineer isnt convinced is no fault but cannot find whats wrong or isnt conclusive?"
I can't get my head around that statement I'm afraid ? If 'we' think there is a fault, we have to keep going until we find it. If there is no evidence of a historic fault and all tests show the circuit to be fine, then there's a good chance the TRC will be passed to the EU.
It really is an in-depth subject as it all hinges on historic line-conditions, DSL performance and what one finds when attending site.