Tokens are not variables, so you can't modify their value. Even a "{TXT:}" token, for example, only retrieves the value of a given variable as text, it does not otherwise represent that variable.
If you want to use dictation, you'll have to use a conditional tree, and a variable to store the actual number.
However, if you use the "Wait For Spoken Response" action, which can do what you're using dictation for with better accuracy, the result of that is stored directly into a text variable.
You'll have to make modifications to your profile either way.
For more information on tokens and variables, you can press F1 while VoiceAttack has focus, which will open VoiceAttackHelp.pdf in your preferred PDF reader.