'Which' "He's waiting for your call at 6 PM Minsk time, which is 5 PM Austrian time". 'Which' because 'which is 5pm Austrian time' is a non-defining clause. It's not essential to the meaning of the sentence - it's only adding extra information. With non-defining clauses, out of 'which' and 'that' we use 'which'.

Everyone in the comments is correct that the right answer is "which." I just wanted to add that you may hear a sentence like this with the phrase "that is," but it will carry a slightly different meaning. The phrase "that is" can mean "in other words" or "that is to say." So if the speaker is saying, "He's waiting for your call at 6 PM Minsk time, or, in other words, 5 PM Austrian time," the person might use "that is" instead of "or in other words." (For this usage, you would put a comma after the phrase "that is.")

I'm Spanish, but I think I can help you. When you say " which is 5 p.m...." you are giving extra information. This info is not necessary to understand the sentence so it is a non-defining relative clause. And, in this kind of sentences you have to use which (or who ) as a relative pronoun. You can't use "that". " This is the dress which I bought yesterday". In this case, the relative clause gives essential information to understand the sentence. It is a defining relative clause and here you can use "that" instead of which (or who). This is the dress that I bought yesterday. One more example: The man who/that won the race is Spanish. Hope that helps. Please, correct me if I'm wrong.

"Which" is what I would say, as a native speaker.

'which' I think.