Thank you for contacting support.
We usually do not allow changing the assigned number to prevent abuse of our numbers on the Individual subscription service.
Can we ask why you want this number changed? We may have plans to support this in the future for our individual Lite/Standard users, however this is still to be decided upon.
Alternatively, if you're on our 'Another Number Teams' (find out more here: https://www.thumbtel.com/teams/), you can buy and then re-assign new numbers. To do this, please sign up to Another Number Teams, you'll then need to transfer in your existing number into the team, then you can delete and buy a new number to assign whenever you want to change it.
Please send us an email at email@example.com with your number, we may be able to help with a solution.