SIP Trunk in simple terms, is the digital version of traditional PSTN lines. SIP Trunks are being provided to connect to your existing PBX which will allow you to make local or international calls thus extending the boundaries of your organization.
SIP Trunk services are highly scalable and flexible to meet different kinds of customer needs such as number of channels and geographic/non-geographic numbers. These can be easily scaled up or scaled down depending on your current demands.
Since SIP Trunk can utilize your existing internet infrastructure, the cost of adapting this kind of technology is relatively much cheaper.