Q: can I turn off cellular access but not wifi?

Take a look at this:

HT4146 iOS: About cellular data usage

You can also temporarily disable cellular data when it’s not needed.

• From the Home screen:
  • iPhone: Tap Settings > General > Network
  • iPad: Tap Settings > Cellular Data
• Slide the toggle next to Cellular Data to off. This will disable all cellular data traffic until it’s enabled again.

If it has access to wi-fi then it will use it.

If you see the wi-fi symbol on your status bar ( the fan symbol) then it is using wi-fi.

iPhone User Guide (For iOS 5.1 Software)

Enable Airplane mode followed by turning on wi-fi access. All other iPhone radios will be off - cellular, bluetooth, and GPS.

You're welcome.

This means you must use the cellular data or internet network to place a call.