Doosra is a virtual mobile service and doesn’t have a dialer option to make outbound calls. However, if you need to call back an unknown number that you’ve received a call from, you can use the call function to do that in an incognito mode- meaning, your number won’t be revealed to the person you’re calling back. You can always opt to call the number back from your personal number. However, this will reveal your personal number to them.