It is extremely difficult to trace a threatening text from a non-fixed VoIP. The phone number from which you received the text message may or may not be related to the sender. It is impossible to tell.
Even reporting this incident may not help because the sender might have a used a free web-based texting service. You can use the displayed phone number to track the owner by contacting your mobile carrier. They even have the ability to report harassment to the carrier of the owner of the phone number.
If you sense some trouble from the threatening text, get the police involved. They can easily get logs from the phone service used by the sender. They may be able to successfully get the ID address of the sender and you can take necessary actions from there.
