Intercept external service call and stub it

Question

I have an external team's package and I want to use one of the functions inside it.

//External package

class A (serviceExternalClient){
    methodB {
        value = serviceExternalClient.callExternalService()
        otherProcesses(value)
    }
}

class A2 (serviceExternalClient){
    methodB {
        value = serviceExternalClient.callExternalService2()
        otherProcesses(value)
    }
}

...

class AN (serviceExternalClient){
    methodB {
        value = serviceExternalClient.callExternalServiceN()
        otherProcesses(value)
    }
}

class A is public and I want to use methodB in all A classes but I don't have access to serviceExternal service so I want to intercept or stub callExternalService to return default values so that otherProcesses works and I can receive its results.

serviceExternalClient is injected using Dependency Injection.

I heard about using Reflection but couldn't figure out how to abstract it.

I ultimately want to do,

A.methodB()

but the external call is blocking the subsequent operation since I don't have access to it. The data provided by external call is irrelavant to me.