Phone-to-phone voip calls is possible for people with IP Phones or SIP devices. They have to configure intervoip for their phone using the following settings:
SIP port : 5060
Registrar : sip.intervoip.com
Proxy server : sip.intervoip.com
Outbound proxy server : leave empty
Account name : your InterVoip username
Password : your InterVoip password
Display name/number : your InterVoip username or voipnumber
Stunserver (option) : stun.intervoip.com