Figured it out. Had to use the Mock.NonPublic.Wrap function as mentioned here.
http://www.telerik.com/help/justmock/advanced-usage-mocking-non-public-members-and-types.html
ultimately, this code snippet worked.
dynamic mockedClassWrap = Mock.NonPublic.Wrap(mockedClass);Mock.NonPublic.Arrange<IEnumerable<OtherPhoneDto>>(mockedClassWrap .GetOtherPhonesByOfficeID(ArgExpr.IsAny<Guid>())).Returns(otherPhoneDtoCollection_AM);