There are two types of VoIP telephony:
Hosted telephony (also known as hosted voice) is a telephone system that resides in the cloud rather than in your office. Users access the hosted telephone system through a standard IP handset and/or mobile app, calls are made and received over an internet connection and are routed to fixed and mobile devices.
SIP trunks – You route your calls using SIP trunks rather than ISDN channels. SiP trunks are best suited for larger businesses as this type of IP telephony can prove more cost effective than Hosted VoIP if there are a lot of users.