Skype can call any subscriber phone line from the moment you put some credit on your Skype account.
The "Skype in" number is optional and meant to receive phone calls from mainly classic telephones and is not required for calling out.
Skype to Skype as from or to computer or pocket computer or Skype enabled phone is (still) free over any kind of internet be it 3G or Wifi or ethernet connected broadband.
Skype over 3G can possibly yield amazing, better than telephone quality. I have tested it.
The video chat function can be fun to surprise some people who are still in the Skype happens on my computer only phase with your outdoor location.

Using the built in SIP client with one of the many available SIP providers works great for me.
You can use different suppliers for different destinations.
Some of them will allow you to call out to fixed numbers (almost) for free.