No, you shouldn’t. Instead, you should see a topic update 
This line
exten => _X.,1,AGI(/etc/asterisk/agi/agi-mqtt/mqtt,/etc/asterisk/agi/agi-mqtt/mqtt.cfg,calls/doorbirdcall)
will send the calling extension to the topic calls/doorbirdcall, so when using for example mqtt-spy and subscribing to the topic calls/doorbirdcall, you will receive a message {"extension":"<extension>"} where <extension> is the number.
To test this from command line, issue this command:
/etc/asterisk/agi/agi-mqtt/mqtt /etc/asterisk/agi/agi-mqtt/mqtt.cfg calls/doorbirdcall 1234567
and press <enter> a second time to end the command. This will publish {"extension":"1234567"} to calls/doorbirdcalls