The general porting rules apply, 

For the port take effect Telkom has to make an appointment to visit your offices and remove the PRI interface from your premises. This can delay the porting process.

A PRI is simply a line offered by Telkom that gives you 30 channels (voice line) 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.

PRI 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 250 of its 300 PRI numbers now and request to port the remaining 50 numbers 2 weeks later. On completion of the porting request, the customer forgoes these numbers and has no right to them anymore.

If you wish to keep the numbers available for porting later Telkom requires you to leave a minimum of 100 numbers active on the PRI. 

If you are under contract Term for the PRI 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. Talk to Telkom for further clarification if you are under contract.