This could be a network related issue.
If using a SIP account, disabling or enabling STUN could help.
• - rport and stun
Please try to enable or disable "STUN" and Rport for your account.
"STUN" ands Rport are located in "Settings"->"Accounts"->your account->"Network Settings".
Please try allowing the microphone usage in the iOS settings:
IOs Settings -> Privacy -> Microphone -> then enable Zoiper.