For telnet we really only have a logical connection to the pseudo /dev device and as such the IP address is not known.
For a raw TCP server (i.e. OPEN(nn) "[TCP];port") once the user is connected you can get its IP address from KEY(nn)).
However it should be mentioned that the IP address you will see will vary based on your network setup. For example if my home PC connects to a machine at our office the IP address returned will be the same for my PC or any PC in my home that connects will come from the same Internet address. Same for VPN access or any number of other network steups.