I am curious as to where you live. 'Old' flip phones do not work on current networks, they only work on older networks. Current networks use LTE for voice, a 4G technology. In the US and Canada the 'old' 2G networks were CDMA and TDMA and AMPs radio technology networks. Those networks were decommissioned between 2018 to 2022. In Europe and other parts of the world, TACS and GSM was the 'old' technology and those networks were decommissioned between 2017 to 2022.
As a point of reference. VoLTE (Voice over LTE) was standardized world wide in 2010 to 2011 and the first VoLTE network went live in 2012.
The first flip phone was released in 1996 and was the Motorola Star-TAC and various models would support TDMA, CDMA and GSM, the 'old' 2G standards.
Flip phones that would work on current cellular VoLTE networks became available in 2013. I guess that might be considered 'old' but it depends on your reference point. For me, my HP-35 calculator that I bought in 1972 for my engineering classes would still work if it were not for the dead batteries, but my Motorola StarTAC is useless. These are old but non-functional, but my HealthKit Oscilloscope I bought in 1974 is still functional.