Option9 service works reliably with any SIP 2.0 compliant soft phone. Here are some softphones that we can recommend:
Zoiper - Free for basic functionality or $49.95 one time - zoiper.com
Feature comparison: Products :: Zoiper
A great lightweight free SIP client with a free version that supports the vast majority of most users needs. Also has an option for a paid upgrade to unlock many high-end features. Extremely easy to setup and use.
MicroSIP - Free - a super lightweight SIP client - not the best UI, but very full featured and very free.
MicroSIP - lightweight VoIP SIP softphone for Windows - Official Homepage
Bria - Free trial and $4.95/user/month, or Bria Solo/downgrades upon trial expiration
Formerly X-Lite, Bria is a great softphone at a reasonable monthly price. X-Lite used to be the standard for free SIP clients, but went commercial -- you can use any version from their website, and if you do not buy a license it automatically downgrades to the Bria "Solo" version.
#1 Softphone for Single Users - Bria Solo | CounterPath
Linphone - Free
A multi-platform opensource phone. Runs a bit heavy due to being Qt-UI based, but full featured and functional.
https://new.linphone.org/technical-corner/linphone?qt-technical_corner=2
