This is what I found:
1
Motorola StarTac Flip Phone Rare | eBay
Motorola StarTAC phones (released 1996) generally no longer connect because they rely on 2G analog/GSM networks that have been shut down in most regions, particularly the US. If in a region with supported 2G, ensure the antenna is secure, clean the SIM card contacts, and use a full-size SIM, as modern networks may not recognize the device
Wow, makes sense. Only 3G onwards are apparently active.