If you are using the example code verbatim then it is more likely to be a PBX configuration or external factor. The first thing to check is that a keevio call works on the same PBX, using the same user and in the same browser. If not then refer to your reseller or installer as that would be a configuration issue with the PBX.
If that works then close the keevio copy. Depending on the firmware version, trying to use keevio - which uses RTC phone, and the same functionality in an API instance can cause one of them to fail.
Which browser and version are you using? (the RTC functionality only works in Chrome and recent Firefox).