BIS_fnc_kbTell's wiki page (and the description for kbTell function) says that radioMode parameter can take a custom radio channel to play the conversations from bikb files.

However if a custom radio channel is created and it's ID given for that parameter, the conversation is played in this radio channel but it also shows an error related to fn_kbTellLocal.sqf used by this function.
Using the ID shows this error:

While inserting it in any other way or giving the channel label as a string shows this:

Quoting user NikkoJT:

kbtell, the command, does support numbers for custom channel IDs, and this is what the kbtell functions end up doing, internally. If you pass a number, it will reach the internal kbtell and should basically work. However, fn_kbTellLocal also uses the channel parameter for some other stuff like subtitles, and in the process of parsing it for that, it goes through a param that doesn't allow all the types that are supposed to be supported - it only accepts strings and arrays, not any of the other things that it could be. And that's what causes the error.