You realy should figure out what VoIP will be used..
Wil it be a SIP app? To a central (internal) server? To a cloud based server? Does every remote site have its own server?
Are the uplinks trusted enough to not be down when the pstn is? If the uplink is down, do you still have inet access?
My advice, first figure out what the VoIP setup will be as it will dictate much of how you will need the WLAN set up.