You might need to edit your number rewriting setup:
First you will need to configure Global Number Rewriting, so your Zoiper will know where you are located.
Please run your Zoiper for Android, then go to Config and select Number Rewriting.
Select your country from the list.
Now you need to go back, select Accounts and then select the account you will be using.
Scroll down to Number rewriting.
You can choose from a list of prefixes or add your own prefix.