Update Set-CsCallQueue.md

Author: CLYVR

skype/skype-ps/skype/Set-CsCallQueue.md

@@ -1331,8 +1331,13 @@ Accept wildcard characters: False
 ```
 
 ### -CallbackRequestDtmf
-The DTMF touch-tone key the caller will be told to press to select callback. This parameter becomes a required parameter when IsCallbackEnabled is set to `True`.
+The DTMF touch-tone key the caller will be told to press to select callback. The CallbackRequestDtmf must be set to one of the following values:
 
+- Tone0 to Tone9 - Corresponds to DTMF tones from 0 to 9.
+- ToneStar - Corresponds to DTMF tone *.
+- TonePound - Corresponds to DTMF tone #.
+
+This parameter becomes a required parameter when IsCallbackEnabled is set to `True`.
 
 ```yaml
 Type: String