Prepaid cell phones make forensic investigation hard. Discuss how can you quickly investigate and collect digital evidence for a crime what involves a phone call (e.g., you checked the victim's cell phone and you find a phone call from a phone number, this phone number could be a VoIP phone, a prepaid cell phone, a landline phone, or just a regular cell phone).
If the victim's cell phone had a VOIP phone call made to him/her, there would be a phone number where the VOIP call was made from. If this phone number is traced back, the investigators can get a contact of the company from which this call came from, and they can give out the IP address of the computer from which this VOIP connection was made. If that computer is an internet café, it becomes harder to trace the caller unless the internet café manager ...
300+ words explain the difficulties in tracing criminal offenses that come with prepaid cell phones.