For the record this might be a better way to go. All of my clients currently have a custom FQDN but the problem with that is when the cert expires your on the hook for getting a new cert and getting the phone system back online. I am starting to think its better to just let 3cx deal with it. I mean who really cares if the way you get to the phone system is "client.ca.3cx.us" or "client.yourdomain.com" yes its nice but in most cases not really necessary.
i decided the same... much easier to pay the maintenance and let 3cx handle the details. My time is valuable. Rather move on to other projects... and - NONE of my users have questioned the domain their phone points to. They probably don't even know.