Often it happens because the "carrier" in question has no "qualify" entry or a "qualify=yes" or "qualify={some number}".
If the round trip "sip ping" exceeds 1000MS (or the {some number} value), the Asterisk system will "disqualify" that sip connection and refuse to dial it. If there is no "failover", the call then fails. It's designed to allow for removing bad quality calls from the system in favor of "failover" more expensive but good quality carriers. But if implemented when there is only ONE carrier ... the effect is not quite as helpful.
Which reminds me: You DO have at least THREE carriers, right? (I mean ... paychecks are on the line here ... it may be considered irresponsible to have less than three if families are depending on this dialer ...
).
Also, I note you posted your Vicidial Version with build, very good, but it is also a very good idea to post your Installer (with version) and your asterisk version.