If you port from one provider to another; with respect to Gekko's queries, if you had airtime with your old provider, you lose it - as in you cant carry it over to second provider.
If you are calling a ported number, there will be a tone warning - to indicate the number you have dialled has ported and cross-network tariff applies. So if you dont want to proceed with the call, you terminate before call set up is completed.
Porting will take 48hours - from the time you do simswap, to the time your new sim becomes active; you will have up to 14 days to port back if you are dis-satisfied; if you dont within the 14 days; you wont be able to port again until after 6 months!
And to port, you will need to pay kes. 170 + VAT. Its instructive to note that a ported number could still be affected by network issues in your old providers network as communication between your old provider and the new one is essential for you to continue getting service.
I personally think MNP is too hyped for nothing; and honestly is most markets after more than 5 years of MNP, only less that 10% of subscribers have ported.
Two Zimbabwe cents worth of my opinion.
The laudable is more often than not rendered laughable by overclaim