phpunit only mock one method of a class

I am writing an integration test and just want to mock a single method in my class to prevent it to make contact with the "outside world"

My class has 2 public methods and I want to replace the method 'dispatchMessage' with a callback, which just gives me back my input value without doing anything further.

My current attempt is this:

        /* @var ChargeReportServiceInterface $mock */
        $reportService = $this
            ->getMockBuilder(ChargeReportServiceInterface::class)
            ->enableOriginalConstructor()
            ->onlyMethods(['dispatchMessage'])
            ->getMock();
        $reportService
            ->method('dispatchMessage')
            ->willReturnCallback(
                function ($message) {
                    return $message;
                }
            );

        return $reportService;

which result in the message: PHP Fatal error: Class MockObject_ChargeReportServiceInterface_fde2b860 contains 1 abstract method and must therefore be declared abstract or implement the remaining methods

What is the correct way to only mock the 'dispatchMessage' method and leave the rest as is?

What works is the following code:

        /* @var ChargeReportService $mock */
        $reportService = $this
            ->getMockBuilder(ChargeReportService::class)
            ->disableOriginalConstructor()
            ->onlyMethods(['dispatchMessage'])
            ->getMock();
        $reportService
            ->method('dispatchMessage')
            ->willReturnCallback(
                function ($message) {
                    return $message;
                }
            );

        return $reportService;

So I replaced the interface with the class itself. Then I disabled the constructor because luckily I didn't need any of the injected objects. And et voilá! I can access my returned value of $message, while the other public method of the class works as intended.