There are several providers for sip telephony that have pure sip services without any cost.
This is possible since sip telephony takes so little resources from a provider.
It is only the call meta information that passes through the providers servers, voice data does not pass thought the provider.
If you also want to call to the old telephone network, the providers normally charge a fee for this.
Here we give information about some SIP providers that provide pure sip accounts for free.