It depends on exactly what services you want to provide for them. Of course, remote extensions could be provisioned, and the users could call in the same manner as others in the office, or a second server could be provisioned. Special VoIP trunks could send any incoming calls directly to these extensions, and any calls they made could be routed out over the same trunks. However, 3CX isn't a replacement for POTS service and any billing, especially long distance, would have to be kept track of. It might just be easier to have the SIP devices, that they planned on using, registered directly with VoIP providers, and have the end users take responsibility for all expenses. As I said, it all depends on what you what to do.