The main advantage of these services is that it avoids high telephone charges (mainly long distance) that are common to companies in the Public Switched Telephone Network (PSTN). Some cost savings are due to using a single network to carry voice and data, especially when users without using the full capacity of an existing network which they can use for VoIP at no additional cost. The VoIP to VoIP calls from any vendor are usually free in contrast to the VoIP to PSTN calls generally costs the VoIP user.
The development of VoIP codecs (aLaw, G.729, G.723, etc.) has allowed the voice is encoded into data packets of diminishing size. The result is that voice communications over IP require very small band widths. Along with continuing progress of ADSL connections in the residential market, these types of communications, are very popular for international calls.
There are two types of PSTN to VoIP services: Direct Inbound Dialing (Direct Inward Dialling: DID) and access numbers. “DID connects the caller directly to the VoIP user while access numbers require that enter the extension number of the VoIP user. The access numbers are usually charged as a local call for those who made the call from the PSTN and free to the VoIP user.
These prices can reach up to 100 times cheaper than the prices of local operators.