Please make sure that the account is not used on another device at the same time. Also, please contact your provider and ask them to allow TCP for your account.
Start your Zoiper for iOs, then go to Settings -> Accounts and open Network features
Note that your provider must support TCP otherwise your account will not register. Please refer to your provider for more assistance
In case it is not possible to use TCP:
Start your Zoiper for iOs, then go to Settings -> Incoming calls and enable "Force background" and "Wi-Fi keepalive".
Please make sure that Zoiper is allowed to show notifications: Open iPhone's "Settings" menu, then select "Notification Centre" make sure Notifications are enabled for Zoiper.