telnet is an insecure protocol. Ideally you should use ssh instead but most which some modern BBS's support both. Of course if you want to dial in on legacy "authentic" hardware then SSH isn't possible.
💡You can SSH (or telnet) from your phone using Termux and it works pretty well (though admittedly not as good without full ANSI support). It doesn't use full height of screen but is still usable. BBS's could be enhanced to support that though.