No, you don't need a key for a third party server. The server certificate is needed  if you are the server.
Normally PhonerLite is the client, because it connects to a server. Normally all traffic takes this established connection.
If you are using peer-to-peer mode, other peers (another PhonerLite for example) can connect to PhonerLite directly. In this case PhonerLite is a server and needs a certificate as server (Take a look here: http://www.phonerlite.de/config_en.htm#Certificates). In this case a private key is needed too.