Share Imacs wifi network connection with iphone

Now I have a problem that is contrary to all I have seen so far. My Imac is often with me in places where I can have wifi network if I declare my MAC address in advance. When I am there, I often want to have network on my Iphone as well, but cannot share that connection because wifi is already used for getting into the net. How can I do access the internet from my Iphone (example: skype or Viber calls from abroad to home) in such situations?
MacBook Pro Mac OS X (10.6.7)