Actually it's not a question.
It's that I want to help people who use Linux and Zoiper.
I had a hard time trying to make my OVH VOIP service work and I tried many soft-phones.
Zoiper3 works. It uses pulseaudio correctly. I have PulseAudio Equalizer installed because I'm hearing impaired.
Zoiper5 doesn't use pulseaudio correctly.
You find Zoiper3 here: https://www.zoiper.com/en/voip-softphone/download/classic#
The file is Zoiper_3.3_Linux_Free_32Bit_64Bit.tar.gz
I have STUN enabled and the server is stun.zoiper.com, even though the SIP server is sip4.ovh.fr.