FTTH and voice issues

Hello,

Will try and keep this brief so its missing details but I hope the gist of the post gets across.

I work at an ISP selling FTTH on a 100% fibre network (from router to core). The entire network is fibre with voice services from a RJ11 phone port from our router for normal phone services. Calls made from a analog phone are routed to a Asterisk server on our network then out to the carrier.

In one case, a customer was unable to see the SSID from the router (router was replaced but issue persisted). After an outage, the fibre from outside was replaced (fibre was cut) and now customer can see the SSID. In another case, replacing the fibre (fibre cut) resolved an issue with noise on the line when on the phone.

The problem is not being able to accurately troubleshoot these issues as there can be a lot of variables. With two identical connections (same switch, same drop-point, same router, same firmware, even the same phone), one might have crazy voice issues (DMTF and calls cutting out) while the other is perfectly fine. Its not to say that the actual fibre itself can cause these issues but I do not have the knowledge to be 100% on this.

I'd like to know how much the actual transmission and frequency on the fibre actually affects customers. For example, how would a 0.10- intermittency in Dbm (both Rx and Tx) affect voice/data services? We have this kind of intermittency with some customers but zero issues reported.

If theres anything out there with info about fibre to analogue and voice/data It would be great. I can't have fibre replaced to test this as management will question it, and I'd need more info before making the case. It might also be that I'm looking entirely in the wrong direction but I hope /r/networking can advise me on that.