I thought maybe it’s possible to create an app as an intercom and using an old app as a button instead of ring etc. (who doesn’t have an old cell at home) the idea which ** I don’t have a clue how to create is: ** that the screen will be locked and on it will have a “button” which will call the house inside every phone has a cam and a mic…
and maybe to lock the phone somehow with a special frame (printed in 3D printer… )
I don’t use them, but I think most of what you want can be found with Tasker and BigTalker.
Depending on which country you are in and the exact model of the phone, there are literally dozens of apps which can act as an intercom on an android phone. 
some of these are called “walkie-talkie“ apps and others are marketed as baby monitors
The ones with are marketed as baby monitors are more likely to let you open up the video on the camera to see who’s there.
Features vary somewhat, so you just have to read carefully and look at reviews.
None of these will integrate directly with smartthings, of course. If you want to do that, you may end up having to use a combination of Tasker and sharptools along with your desired intercom app.
So, yes, it can be done. Buying something like ring will give you a lot more features in a weatherproof package with a battery power option and better integration. But if you don’t want to spend that money you can start playing around with the app possibilities.
Alfred seems to be one of the most popular apps and includes both motion detection and two way audio, but it has a $30/year subscription fee. And you have to pay even more if you want to turn off ads in the app. But it is cross platform, which means you can have an old android phone at the door and then use the app on an iPhone that’s your every day phone, or vice versa. Even so, it’s very highly rated, in part because of the motion detection feature.
(If your old phone happens to be an iPhone, there are some app possibilities for that as well. See the CNET article above.)