Porting out an exchange number from one company to another is quite painless. The original setup is complex but now most of the exchange data is in place it is short & sweet.
There is one potential problem with ported numbers. Should the Original exchange from which the No is ported has a catastrophic failure all calls TO the ported No will fail. Outgoing calls will be ok.
All ported calls are first routed to its original exchange which hosts the number range in which it lies. It is then bounced back into the network as not mine with an identifier prefix to route it to the new destination exchange. Hence an original exchange fault can cause such faults. However exchanges are pretty reliable and the only troubles I saw were caused by mains power faults.
