I did similar in my apartment in Amsterdam but a little more low-tech. I soldered the relay on an Esp8266 directly to the unlock button on the intercom PCB in my apartment. Worked flawlessly for years
The use of a phone number rather than a username seems like a BIG downside.
I’ve moved countries several times and as a result been forced to change my number several times.
This causes a huge headache with any service that relies on phone numbers, especially two-factor etc. Phone numbers should be thought of as ephemeral while usernames can persist.
Is there not an inherent risk using an AWS service (Route 53) to do the health check? Wouldn’t it make more sense to use a different cloud provider for redundancy?