We recently reviewed Zoiper 3 for Windows, a softphone client for PCs that offers both free and paid versions. We were impressed by the low resource consumption of the app during voice and video calls, as well as the support for faxing.
Here's an excerpt from the review:
Zoiper for Windows is a satisfactory softphone client for Windows. The interface is simple and the search bar makes it easy to find contacts. The free version offers standard call handling functionality, but you will need to upgrade to a premium account for features like call recording, call transfer, and conferencing.
The two features of Zoiper for Windows that stood out the most were fax and low resource consumption. Not having the G.722 codec was a bit disappointing, but Zoiper says it will be included in the next version, Zoiper 5. While Zoiper 3 for Windows is capable of basic everyday calling, we would recommend the OnSIP app or Bria 4 for a fuller feature set for business use.