I came across an article about how the SA government was tracking women in hiding by tracing their IMEI number on their GSM cell phones. IMEI is similar to a MAC address for mobile and is tied to a device regardless of SIM card changes.
On CDMA phones there is an equivalent, the Mobile Equipment Identifier (MEID) string.
https://en.wikipedia.org/wiki/International_Mobile_Equipment_Identity
https://en.wikipedia.org/wiki/Mobile_equipment_identifier
Looks like it is possible to change an Android divice's IMEI with some work (https://itechhacks.com/change-imei-number-android-devices-easily/).