Most fax machines will work on VOIP networks and with Cloud PBX. An analog adapter is required to convert the fax signals from standard telephone signals to VOIP.
SuperG3 fax machines (AKA high speed fax or 33.6Kbps fax) are known to not function properly over VOIP networks. This issue is industry wide and not specific to Cloud PBX. The symptoms of this issue are blurry pages, mid call fax drops or complete fax failure. In some cases where a SuperG3 fax machine communicates with a non SuperG3 fax machine, fax calls could complete properly, because the lower speed fax machine causes the SuperG3 machine to throttle down to a lower speed. As a result, problems with SuperG3 fax machines could appear to be intermittent.
If you have a SuperG3 fax machine, it must be configured to transmit and receive at 14.4Kbps. If the machine speed cannot be reconfigured, it may not be supportable over VOIP.