Some Iphone apps suggest there is no phone calls costs when using the net.. be warned 

if they say the call is using a Data plan (that might be a whopping cost if your “Roaming” ie., overseas from your base – so check with your service provider)  ie ., ask them what is the consumption rate for a call on a data plan

I think a phone call is around 8kb/s to 20kb/s which is around 1mb per minute and most roaming data plans are aud$20 to $30 per mb  ie.,, data plan could be $30 per minute if international roaming.

If your international roaming (o.seas from your base) 
get to a wifi free zone, turn your phone on airplane mode (turn off the 3G) and then your wifi free only and make your calls on skype.

ofcourse if you are local within your country the 3G cost is (should be) peanuts compared to phone costs

Best Practices - Overseas Travel 
1. call your service provider and get the facts and ask how to minimise and record their name and details
2. in settings, general useage - pre reset before you go to clear the memory, then anytime you can check your download useage (used to far)
3. turn the phone to airplane mode (settings, airplane)- it forces the 3G/net to be disconnected 

make sure you have installed skyp mobile - you can talk to skype users on the net free from mobile to desktop and send sms
if you are travelling - check out these best iphone apps http://www.networks.auswathai.com/Home/best-iphone-apps  - you can use them at wifi-free venues

anyone got suggestions please add