Sending DTMF codes after answering of a call. SIM800L

**Describe the problem you have/What new integration you would like**

**Please describe your use case for this integration and alternatives you've tried:**

**Additional context**

It would be nice to make a SIM800 module that listens for incoming calls from the entrance phone, that is linked to a ordinary phone number. As with many entrance systems, it is used by answering the call, listening who is there and pressing "5" on the phone keypad to open the gate.

This would need an option to send DTMF tones after answering the call. The command is the same either way `AT+VTD=x` where x is *#1234567890 (and ABCD?) This command is optional, and is used to set the DTMF tone duration in 100mS steps: `AT+VTS=x`

Start by locating the SIM800L call-handling integration and any existing support for sending AT commands after a call is answered. Check the module documentation for AT+VTD and AT+VTS, including supported DTMF characters and timing. Done means an incoming call can be answered and a requested DTMF code, such as 5, can be sent reliably.

Written by the indexing model from the issue text.