IMHO, I think you need to get Internet speed out of the customer's head when talking about VoIP. I have not seen issues related to the Internet bandwidth in a long time It is always the Quality of Service. When the Internet speed becomes an issue is when they are doing heavy uploading, such as cloud backups. The upload speed is almost always slower.
I have seen new problems arise after the customer upgrades their internet speeds. I had one user repeatedly tell me that he should have plenty of bandwidth because he just upgraded his internet speeds. All the issues happened after the speed upgrade. VoIP was working just fine on the slower speeds.
Another customer had a Unifi USG router. Issues after the Internet upgrade, in this case, were likely due to the router bottleneck. I disabled all the extra features, like intrusion detection. That fixed their problems. The router did not have enough throughput to handle the faster Internet speeds with all the security options enabled. A lot of these business routers have a throughput of about 300-500 Mbps when you start to enable extra AV or other security features.
Packet loss is the real killer of VoIP.
This is another nice tool to visually show the customer the ISP issues:https://www.pingplotter.com/