Acrobits Softphone is the leading SIP Client on the App Store, featuring push notifications and the G.729
In both cases, a SIP-compatible softphone, PBX, or softswitch is configured to place certain (or all) outbound calls through the SIP provider; likewise, the switch or softphone is configured to register with the SIP provider to be notified when a new inbound phone call is available to answer.
Voice Operator Panel (VOP) is a professional SIP softphone for operators and receptionists with Outlook/LDAP/XMPP/MSN/CRM integration, built-in web browser and e-mailer.