PXO+

PXO+ is a very flexible softphone with hundreds of features including:

Softphone for RTC Quality Assurance and End Use
RTC and VoIP signaling via SIP and SIPs
RTC and VoIP media via RTP and SRTP
ICE, NAT and STUN capabilities
Speech codecs: G.711 MuLaw, G.711 ALaw, G.726, G.723.1, G.729A, GSM6.10, AMR, G.728, G.722, iLBC, Speex, EVRC, GSM-EFR, G.729E, G.729D, LPC, G.722.1/RFC3047, AMR-WB/G.722.2, RTAudio, AAC-LC, SILK, G.718, Opus, iSAC, EVS
Video codecs: H261, H.263 (RFC2190,RFC2429), H.264, H.265, Theora, MPEG-1/2, MPEG-4, Opus, VP8.
Signal generation and detection
Scripting and VoiceXML Support
T.38 Fax
STIR/SHAKEN