SIP client (intercoms), Asterisk and native App, receiving notifications and calling offline clients, how to implement it?

Question

SIP

I welcome everyone!
We are developing an application for a tenant of an apartment building, and one of the functionalities concerns working with intercoms using the SIP protocol. We raised Asterisk, added several clients (MikroSIP) and from the intercom it turns out to get through to clients and vice versa (not always), but if the client is Offline, then the connection is immediately broken. This situation can also be in a native application - if the application is not active (minimized), then for Asterisk the clients are offline.
Questions:
1. How can I receive notifications to third-party systems about a call to a subscriber from an intercom?
2. Is it possible to create virtual SIP clients that will always be online?
3. Perhaps there is a ready-made solution for the SIP client (for example, under React) to implement it on the front and implement the transfer of sounds from the client to the intercom via Asterisk?
4. It is also possible that there is already a solution for SIP clients on a VPS or a cloud that receive a signal from one client and wait for a response from the second, and if the handset is not picked up, then timeout or cancellation. And we received data from this service via API.

I plunged into SIP technology 2 days ago ... and there is a whole ocean.

Any advice on where to go or dig would be welcome!

Thank you!

Answer 1

1) Asterisk has a bunch of different interesting APIs and integration methods. The dumbest thing is to set up CDR export to the Postgres database and hang a trigger there that will send a notification wherever you want.
2) Pornography and misunderstanding of how telephony works. This does not need to be done, because there is point 1.
3) Eeeee, clients for androids in particular and millions of platforms are full. There are also clients for the web, but they have a drawback - a minimized browser freezes the page and scripts on it, idea 0/10
4) Still pornography, similar to point 2
In short, you should hire a person who is well versed in telecom, asterisk and IT architecture, so as not to produce digital freaks.
Your main problem is that you want instant message delivery to a user's phone. BUT!
This can be done relatively normally only with the help of push notifications, and this is not a fact - modern phones have such fat processors and programs that they freeze everything they can to save the battery. So if you want your application to be constantly online and ready to respond, you need to force the user to go into phone settings and cancel power-saving restrictions.
Have you ever heard the passage "I have xiaomi and notifications do not come to the cart"? That's it - cunning Chinese cut all applications in general (especially if the user always sits in the "battery saving" mode).
Now, about your bad idea to spawn virtual SIP clients (neighed by the whole office). You already have an asterisk to which intercoms are registered.
The asterisk dialplan allows you to do at least a bald thing - pull the database, call lua scripts, even call binaries. Well, the scripts in the language of the asterisk itself should not be ignored either, there are a lot of smart things there, like an auto-answer, voice mail, blablabla.
So the idea is this - smoke mana from the asterisk and vacancies on the headhunter for elders in the asterisk. To hell with a self-made application, integrate into messengers - usually people are interested in making messengers work for them, so they google (or turn to an uncle in a shopping center with a sign "setting up whatsapp 250 rubles") how to make it all not fall asleep.
The worst thing that can happen to your intercom and telephone service is if emergency services go to a person, and something goes wrong and the services don’t arrive, you will have questions.
Because the only service that guarantees (formally and legally) communication for emergency services is the good old telephony (and the trumpet in the hallway, which is connected with the good old copper wire directly to the intercom). And all these VoIP were playing - that's right, frivolous jokes.