1. Will users be able to chose their numbers or it will be allotted randomly?
At present the number is allotted randomly. We are considering some limited form of choice in the future, for example giving you a choice from a list of numbers that we have available to us that have a prefix/substring/suffix chosen by you.
If you mean whether you can get a number that you previously had again at a later date; this is just a special case of question 1 above. If we implement the ability to choose the number you rent (within the constraints mentioned above) then it will be possible to choose a number you previously rented as long as it's still available.
At any given time, a number is only assigned to one person. However, if you end your rental of the number it can be assigned to someone else. I would not recommend using these numbers for 2FA or other security purposes unless you are confident that you can keep your account funded sufficiently that the rental will not be terminated. It's worth noting of course that this is the same as a real mobile phone. If you don't top your pre-paid phone up for a period of time, or you cancel your post-paid account, the number will eventually be given to someone else.