SIP Error: 480 Temporarily Unavailable

Finally i found the cause of the issue:

For the “temporarily unavailable” error, it is a condition from your VOIP service. It maybe caused by the service is running out of capacity temporarily. It may also be triggered by some limit, such as maximum calls to make in a second for a particular account. Different service providers have different treatment of errors.

You may also try to manually call the phone number. Some VOIP service or PBX system may not report error conditions accurately.

Depending on the exact cause of the error, your solution may vary. For example, you can try to add another VOIP account for different lines (from the same VOIP service or different one), or slow down the calling (by defining dialing intervals in Voicent gateway), or calling during less busy time, etc. Normally, your VOIP service can tell you the best solution.