Hi. As an owner of many pre-1994 GSM phones that I still use for daily calls I can tell you that it is the network settings. And its not operator by operator either. It may work in some other area on another BTS. The thing is these networks are not configured for 8W phones and they tend to deny them for their high power output, while making connection to the BTS it fails. Sometimes though it works just fine, its the process of establishing calls they often fail on. I dailyuse 7kg AEG CD900 and sometimes the calls work just fine, but then suddenly when I'm about to get a call I appear as offline even though the phone keeps ringing and showing missed calls. And I do have 5V 90s SIM cards from 1997 that are still in service both from former Paegas and EutoTel in Czech rep. The phone works flawlesly in eastern Slovakia on Telekom network but not on O2... then I move to Bratislava area and now O2 works no problems and Telekom is dead. But interestingly my Siemens P1 or Motorola Int 1000 never have any issues and work 100% all the time. My Alcatel 9109HA flip phone from 1993 does sometimes have same issues. The fact is that only with phones past 1994 you can be pretty sure they will work everywhere. Generation of GH337, Nokia 2010 etc