How can I receive phone calls from a landlines/cellphone when I have Wi-Fi connectivity, but no cell phone service?
This would give you a real phone number that anyone could ring from a normal landline or mobile, that would ring through to Skype on your phone (or any other device) over your data connection.
T-Mobile US has a Wi-Fi Calling app available, and other carriers might have done the same. There might be charge for some of these services, but when I was on T-Mobile it was free.
Yes if you use a VoIP app and your cellphone service provider lets you divert your number on no connection - but remember that you will bear the cost for the diversion.
If you want to just have a wifi-based (VoIP) number that's separate from your cell number then any VoIP service provider will be able to do that - I have a test VoIP setup on my HTC Desire in the UK that lets anyone call my mobile on an 0845 number.
I you can get a free or paid SIP account and use something like SIPDroid.
I believe that Viber will do just that, if you're calling other people with Viber.