Hello,
Go to Configuration -> Accounts -> Add account -> SIP/IAX -> Select yes if you already have an account -> Manual configuration and fill the required settings for Host, Username and Password. If any additional settings (STUN, Outbound proxy, etc.) are required, fill them as well.
In case you still do not have a SIP or IAX account, you can review some reputable providers here:
https://www.zoiper.com/en/voip-softphone/download/zoiper3/providers