All UMTS networks or newer use TMSI the actual IMSI is only sent once per baseband reset as long as there wasn’t a period where a hand off between VLRs didn’t o

All UMTS networks or newer use TMSI the actual IMSI is only sent once per baseband reset as long as there wasn’t a period where a hand off between VLRs didn’t occur and you didn’t roam to another network without direct peering. That said there is a trick to bypass TMSI for tracking as you can trigger an IMSI re-registration by interrupting mobile data or by forcing the mobile phone to downgrade through jamming. Also TMSI only prevents tracking not eavesdropping, it makes targeted eavesdropping harder but not impossible if you can track the target through other means.