hmm - agreeing to the terms and conditions in the user licence - which you have to do in order to use the service - gives them the authority to use your pc and internet connection as a node in the whole scheme of things. By accepting, therefore, you give someone else access to your computer. In addition - as said previously, the service runs in additon to a firewall, but because of the agreement, you provide a back door in anyway.
your choice really, if you are happy with the free phone calls then great but if you are very security conscious then you won't want to agree with the T's & C's