In version 18 on the ring groups you had options to set a specific ring tone, this option is not available in version 20 unless you upgrade to the enterprize version, I know this because I have just upgraded the problem customer in question to the enterprize version at cost and now the ring tones are available.
3CX have tiered if you like the telephone system and removed features, in hindsight if this was the case we may not have gone with 3CX. I feel this is a bit of a cop out from 3CX, surely all existing customers should get enterprize edition at no cost as the full product is what they essentially opted for prior to any v20 and tiering.
P.S I know 3CX needs to make money to make it worth it to operate but short changing existing clients is probably not the best way to go about it, which this feels to be the case, please let me know if I am wrong on this.