How you have configured your account? Did you use the VoIP providers list or you scan a QR-code? If so, please note that the settings cannot be changed afterwards.
To be able to enable the Number rewriting, you would need to configure your account manually by using the following instructions:
1) Start Zoiper and go to its "Settings" tab.
2) Open the "Accounts" section and select the "+" button into the upper right corner.
3) Select whether you you already have an account or not;
4) Select whether you will manually configure your account or you will select your provider from the list;
5) Choose the type of the account - SIP or IAX.
6) In case you are configuring your account manually, enter the information you receive from your VoIP provider about Domain, Username, Password, etc. into the appropriate fields and select the "Register" button.