If you're concerned about the inbound calls completing reliably, but want to save money on long distance and international calling, another solution is to setup a connection to the PSTN for inbound calls, and route outbound long distance and international calls over an ITSP.

To protect your system against the case where your Internet service provider goes down, you can also setup your system to reroute calls to the public telephone network, offering a layer of security. If this happens, it may cost you more to make calls, but at least you won't risk losing business due to a service outage.

Finally, if you're migrating from a legacy phone system to a VoIP system, you may want to consider keeping your current PSTN connection and simply replacing the PBX. The advantage of this is that you can keep the reliable voice circuit you already have to route calls while reducing the amount of time you need to test and put the new PBX into production. It can be time-consuming and frustrating to install a new data or voice circuit, so if you already have resources up and running, it's best to stick with them.