mock method calling callback function sinon

How do you mock an outer method calling a callback using sinon? Example given the following code, getText should return 'a string' as response in the callback function

sinon.stub(a, 'getText').returns('a string')
let cb = function(err, response) {
   console.log(response)
}
a.getText('abc', cb)

and it should produce the output 'a string' since it calls callback function cb but there is no output