Firstly what is an ITSP? As described in our glossary, “ITSP: Acronym for Internet Telephony Service Provider. This is a company that allows calls to and from ‘normal’ telephone numbers (PSTN)”. Here’s a few things to think about. Call plans – Plans maybe a cost for each call, bundled minutes, or plans for domestic / international usage. Providers offer the ability to phone free between customers in their community, is this something you’ll need?