It can be used as a legacy local server too.
Creating a legacy cloud account is no longer possible.
In addition, the Legacy app will be removed from the app/play stores at the end of this month, so direct new installations will no longer be possible after that date, although existing installations of the app will continue to function, and side-loading will be possible on Android devices.
Also, the current legacy app available in the app/play stores prevents new account creation on a local server, but there are workarounds for this, or you could use an earlier version of the app which was released before new account creation was stopped.
You may be better using a Blynk IoT account instead.
Sending an SMS from the legacy local server is theoretically possible, via a broker service, but I’ve never tried it.
I’m not sure if SMS is possible from Blynk IoT, I don’t think it’s possible yet.
You’ll need to be running Node-Red on your Pi anyway, to allow communication with either your local server or IoT cloud server, and sensing an SMS from there is straightforward, provided you have access to an SMS gateway - which usually needs a subscription.
However, if the internet at the location where the Pi is installed fails then you won’t be able to send data, alerts or SMS messages to a phone.
The only workaround for that would be a device with a SIM card that is connected to your Pi - either directly or indirectly - that could be used to send an SMS.
You should probably start by reading this about using Node-Red with Blynk…
Pete.