Here in the UK there are no charges or limits on incoming calls, outbound calls usually involve a connection fee followed by per second or per minute billing there on. So yes we get unlimited inbound calls but that goes for any telco service here in the UK including mobiles.
Only freephone numbers such as 0800 numbers have an incoming call charge which is currently around 5p + VAT per minute which is billed to the owner of the freephone number. 0800 numbers cost around £4.99 per month via VoIP. 0843 numbers I believe earn you money, some VoIP providers give you a free 0843 number but keep the revenue in exchange - anything around 2p per minute!
https://www.voiptalk.org/products/0843+ ... hic+Number
I'm with Goofy, don't go giving away free unreliable phone numbers. I'm not aware of anyone wanting such a thing unless maybe a temporary number just to test without charge? But then if they call the number during testing and it fails surely that would put them off your service right?
Go for a reliable VoIP service as best you can, that's how Voipfone started. Reliability, reliability and err reliability! One of the biggest and best UK VoIP providers around today!