Why we need third party agent phone software?
There have many software for call center server.
Eg. MicroSip, Zoiper, X-Lite etc...
It allowing to do high quality VoIP calls (person-to-person or on regular telephones) via open SIP protocol. From cloud of SIP providers you can choose best for you, register account and use it with MicroSIP or another . You'll get free person-to-person calls and cheap international calls.
It is the main feature for Third Party Sip Phone.
But we are using with call center.
You need to add sip account(create from call center server) at Third Party Software before using.