I have the exact same issue. I hoped that there is s problem with the settings , but in the the Advanced Settings- Supported networks all options are checked.
It is not about speed because if I have another phone around and I make it a wi-fi hub and connect the phone with zoiper installed to its wi-fi the other end can her me.
I tend to believe that there is a software bug. Zoiper if you are reading this please fix.
I am a paying customer, I have the GOLD package with additional audio codecs.