Inputmodes depending from number of retries

Hello all.
I'm developing a vxml application where the maximum number of retries in 
getting user answer is 4, but the first two times user can answer only by 
dtmf, and the third and fourth times only by voice. 
Have I to use two different <field> (one with inputmodes=dtmf and the other 
with inputmodes=voice), or is there another way to to this ?

Thanks a lot for your help
	Clara