compudoor
No, there is no cancellation window in which to cancel without the commitment or being required to pay an Early Termination Fee . Like the HughesNet service itself, the 24 month commitment is active the second the service goes live.
Also, please keep in mind that, although 3rd party Voice services often don't work with HughesNet, HughesNet's own Voice service is specifically designed to do so.
Edit: I should add that it's actually 45 days from the date of cancellation that you have to return the ATA. If they don't get it back within this amount of time they will charge a $75 un-returned equipment fee. You can see the legal particulars for the Voice Service, including the info I have given, here.