Zoiper softphone

Zoiper (formerly idefisk) is a softphone that supports IAX and SIP protocols. Proprietary. There are paid and free versions. There are versions for Linux, Mac OS X and Windows.
Zoiper is compatible with SkyPBX platform, examples of settings:

Instructions for connecting to the SkyPBX platform for Android

Instructions for connecting to the SkyPBX platform for iPhone

Any available versions are available here