It support all SIP based VoIP services ,Such as SkypeConnect, Vonage, CallWithUs, CallCentric, LocalPhone, ViaTalk, RingCentral and so on.
If you have IP PBX system (Such as Asterisk, Cisco, Talkswitch and more), you can work with it too.
If the call volume is large, you can use the SkypeConnect.
If the call volume is small, you can try the CallCentric, CallWithUs, RingCentral and so on.