Downloading and using Zoiper 3.7 is legal if you own a valid license for the Pro version or if you are using the Free version as per its original license agreement. However:
Because 3.7 is old, it struggles with modern NAT traversal. You must configure the STUN settings manually if audio is not working. Zoiper 3.7 Download
: Many Linux users find that Zoiper 3 handles PulseAudio more reliably than later versions. Downloading and using Zoiper 3
(Recommended)