Subsequent ajax calls in angularJs

I'd like to make subsequent ajax calls in angularJs inside a service.

I've tried with something like this:

var requests = [
    {'url': 'call1'},
    {'url': 'call2'},
    {'url': 'call3'}
];

return $q.all([
    $http({
        url: requests[0],
        method: "POST"
    }).then( /*callback*/ ),
    $http({
        url: requests[1],
        method: "POST"
    }).then( /*callback*/ )
]);

But this make alla ajax in parallel. I need a way to make this calls subsequent, so after first end, it calls second....