IBM ISAM (enterprise access manager) was doing this 5+ years ago. The prompt for one time code will look like this [1]:

1234-_______, and email/sms will have two numbers 1234-554566.

Don't think they explained the reason for the first part in the message though. Just highlighted it in a different color.

[1] https://philipnyecom.files.wordpress.com/2017/02/otp.jpg