The general porting rules apply.
A BRI is simply a line offered by Telkom that gives you 2 channels instead of one, they generally packaged with a set of 30 numbers that can be used for marketing purposes or direct dialing to each employee.
BRI number/ numbers (normally 1 Main Number with a sequential set of numbers following the main number) – Any number that is not ported will be recycled by Telkom and no longer available to the customer. e.g. a customer cannot port 25 of its 30 BRI numbers now and request to port the remaining 5 numbers 2 weeks later. On completion of the porting request, the customer forgoes these numbers and has no right to them anymore.
If you are under contract Term for the BRI Telkom will not allow you to cancel it. So you may port the numbers but you will still have to pay Telkom for the remainder of the contract term.