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.
If you don't have a VoIP account yet, check some reputable providers here:
https://www.zoiper.com/en/voip-softphone/download/zoiper3/providers