According to a discussion on the mailing list:

If you check filter_get_bug_rows() in mantisbt/core/filter_api.php (v1.2.8), it is mentioned in the comment that you can pass -1 to retrieve all the issues in a single page. So I did the following call and it was successful:

mc_project_get_issues($this->bt_user, $this->bt_password , $this->bt_project, 1, -1);

It would be nice if the comments from the original api is copied to soap-api.