If you remember, there were stories in the news a few weeks ago about Russian aircraft violating Turkey's airspace. Turkey issued a strong warning at the time, and the stories disappeared for a while.
I don't think Turkey had any obligation to warn them again.
... the Turks are obligated to attempt to force the offending aircraft out of their airspace.
How do you do that?
How do you do that?
We (aviators) maintain multiple globally known frequencies (in the US and NATO they are known as "Guard" 121.5 and 243 MHz) that communication can be maintained to avoid situations like this.