http client 请求出错

python 3.6.2 ，应用作为客户端，用 aiohttp 方式访问 http服务器，代码如下：

async def call_phone(self):
    data = """
    <?xml version="1.0" encoding="utf-8" ?>
    <Transfer attribute="Connect">
        <ext id="216"/>
        <outer to="1871234567"/>
    </Transfer>
    """
    headers = {"Content-Type":"text/xml"}
    async with aiohttp.ClientSession() as session:
        async with session.post(self.phone_uri, data=data) as resp:
            resp_data = await resp.text()
            print(resp_data)

运行报错 aiohttp.client_exceptions.ServerDisconnectedError: Server disconnected