- Added `buttons` field to `OutputDTMFFrame` and `OutputDTMFUrgentFrame` for sending multi-key DTMF sequences as a `list[KeypadEntry]`. Use `OutputDTMFFrame.from_string("123#")` (or the equivalent on `OutputDTMFUrgentFrame`) to build one from a dial string, and `to_string()` to convert back.
