IM+ uses SkypeOut technology for making and receiving calls. It is cheaper, better quility and can't be interrupted by an incoming call over your phone service. Thus you need to have SkypeOut on your account to make and receive calls. You can make a test call to a test account echo123.
To save on calls you can redirect your callbacks to any land line you have nearby yourself. So the charges will be minimum.
You can find some more information on our site IM+ for Skype :: BlackBerry :: Products