The single-key `button` field on `OutputDTMFFrame` and `OutputDTMFUrgentFrame` is kept as a first-class ergonomic shortcut for the common single-keypress case, equivalent to `buttons=[button]`. `buttons` takes precedence when both are set.
The single-key `button` field on `OutputDTMFFrame` and `OutputDTMFUrgentFrame` is kept as a first-class ergonomic shortcut for the common single-keypress case, equivalent to `buttons=[button]`. `buttons` takes precedence when both are set.