Investigating non-fixed VoIP (Voice over Internet Protocol) numbers generated by burner apps or Google Voice requires a combination of technical digital forensics and legal processes.
Investigators begin by performing Line Type Lookup and Carrier Identification analysis using specialized database tools to determine the underlying provider (such as Bandwidth.com, Google LLC, or TextNow) hosting the virtual line. 
Once the service provider is isolated, licensed investigators or law enforcement utilize formal legal channels—such as subpoenas, court orders, or ex parte motions—to compel the platform to release underlying subscriber records.
These provider disclosures reveal critical digital footprint data, including the account owner’s registration email, credit card billing details, forwarding phone numbers, and account creation IP addresses. 
Finally, investigators cross-reference those IP logs and linked user credentials against proprietary skip-tracing databases and ISP records to pinpoint the target’s true physical location and real-world identity.

Kylesinvestigation.com