Basically, VoIP calls offer the possibility for better voice quality compared with traditional PSTN calls. In case both sides support the HD voice standard, the bandwidth used is higher and voice quality better than with traditional analog or ISDN calls.
In addition, when connecting our system for example to callcenters, we experienced that using a direct VoIP Trunk eliminates many points of failure compared to using the public telephony network. A VoIP Trunk gives us complete control over the used codec (bandwidth) used for the calls. As a backup in case of failure, we configure a fallback mechanism to PSTN connection in case the SIP Trunk fails.