As you mentioned earlier, different icons are used in different apps. This makes it difficult to determine a generally understandable standard. I remember Jakob Nielsen saying, "Users have spent most of their time on other websites." With this attitude, Here's what I would do::
- Identify the target user group for the app you are designing for and
- Understand which messenger / chat tool you are most likely to be used to. then
- Design symbols that are very similar to those of the most popular messenger for the target group
WhatsApp appears to be the world's most widely used messenger [data] by number of users. However, if your target audience is primarily based in China, WeChat will most likely be used frequently. So you can use WeChat to inspire your icon designs. These designs could be tested with real users to make sure they understand the interactions supported by the symbols.
You can also view text only as an alternative to symbols or symbols in connection with a text label.