With ten worldwide locations with each only having a very few users, I would suggest you look at Kamailio/Siremis, it is a SIP proxy that is easily clustered, and supports sip connections for local PSTN connectivity (or dahdi if so fits your needs) and has all the basics of IVR voicemail and media bridging/conferencing, all the HA stuff can be done over IP and BGP.
We do offer a SaaS version of our PBXact program that may work for what you are doing, feel free to contact us in the Sales Department to review that program, it’s not multinational, but can easily be used virtually or on premised based equipment. You can open a sales ticket at support.schmoozecom.com to have someone contact you directly.