In accordance with fcc guidlines,
Fees and Charges
•Companies may assess fees to recover the costs that they incur in providing number portability. Fees may vary between companies, and some companies may not charge any fees.
•Companies may not refuse to port a number because a consumer has not paid for porting.
•When considering a switch, consumers should ask the new company whether it charges any number portability fees and whether those fees can be waived.
http://www.fcc.gov/cgb/consumerfacts/numbport.html
I remember when Local Number Portability (LNP) started back in early 2000, only applied to wireless to wireless transfers. At that time some carriers were charging recovery fees, allowed by the FCC, for meeting compliance requirements of LNP and others (E911 for example).
For Landline and VOIP, I'm not sure what recovery method, if any, are being used for LNP.
Bottom line....it's a Federal Mandate so they have no choice.
If it were the carrier's option, LNP will not be possible since a number which is not transfereable help to retain the customer on their network. The coster for business cards, letter heads, phone number association with you business was a big reason why people had to keep their phone numbers.
Carriers do have options to help recover the cost for implementing LNP and continuous operation of LNP. Once your Ported number is in the routing table "database", that's is it.
The costs are already being charged to you and me through fees and taxes. You are not getting porting for free. They are not losing anything, but a customer. It's also a two way street> Port In and Port Out.
The drivers on whether a Carrier keeps a customer are the Cost and Quality.