You can use Sipgate to get an incoming telephone number for free. For outgoing calls it is less expensive to use intervoip especially for abroad calls. But from Intervoip you don't get an telephone number. To use that Siemnes Gigaset C47H you have to buy an VoIP analog adapter.
But it is better to use an VoIP phone. A cheap one is the LIP TA 100 phone or an VoIP Phone from Gigaset are very good. It is also possible to use an old DSL Fritzbox as VoIP gateway that is really comfortable because you can define rules to use different VoIP provider for different country's or to mobile phones.