21206: Invalid SendDigits

You attempted to initiate an outbound phone call, and you included a SendDigits parameter that wasn't valid. SendDigits is optional, and specifies that Twilio should simulate sending keypad digits when the party answers. SendDigits may contain numbers, the # and * characters, the letters A, B, C, D, as well as lowercase w to "wait" for half a second or uppercase W to "wait" for a full second.

The parameter SendDigits includes an invalid character.

Ensure the SendDigits parameter only contains the characters: 0123456789ABCD*#wW.