YouMail - YouMail turns visual voicemail into a personal assistant for handling all your missed calls.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Hi Voicemail - Stop robocallers filling up your voicemail
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
MightyCall - Virtual phone system that lets you run your business from anywhere.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.