Compulsory involves conscription or a draft, I'm not sure how the "jail or army" thing is classified but since you are given a choice it's technically stelly voluntary.
The "professional" classification of the army doesn't mean it's voluntary professional means that you can be a career soldier and you are paid for doing so even if you are drafted, conscripts on the other hand are not paid other than a small stipend.
That said once you sign up the contract this isn't a job, you can't quit, you can't "call in sick", can't say no to anything, you are effectively forced to give up certain rights for the duration of the contract.