There are two telephone line formats: FSK (International Standard) and DTMF. Imported cordless telephone caller ID only supports FSK, and the line formats in different cities are different. The way to judge your own line format is: first, adjust the time of your own phone, make a home phone call with your mobile phone, and let it ring for a while before the mobile phone hangs up. Then look at the time of the call. If the time will come back automatically, the line is FSK. If not, it is DTMF. The reason is that FSK standard contains time signal, while DTMF does not.
According to the experience of selling cordless phones, almost 95% of the lines in Shanghai are FSK except ISDN single-wire communication.
If you are interested in cordless phones, you can come to me.
/czxzx