the article?

You’re right, since you’re referring to a specific phone call. But English is not a precise language and the article “the” should probably be followed by something else such as, “I have to take the phone call on my other line.” Or I have to take “the phone call coming through.” Language is subtle and confusing, even for first language speakers. That’s why you see so many errors here. I’ve gone on the French site and surprisingly, at least to me, people make the same kinds of errors in French that English speakers are making, so it seems that language problems are universal.

You don’t say, “I have to take a phone call”, do you? I usually say, “I have to take this (call).” I can see why you would ask, though. This situation fairly cries out for the definite article, doesn’t it? Nevertheless, “the” is not used here. The convention (and that is all it is) is to use “this”, perhaps to distinguish this call from any other calls you may receive that you may not wish to answer. Good question!

Because phone calls are generaly referred to in the plural because there is logically the possibility of more than one. If you say “taking the phone call”. I’ts like there is only one phone call in the world. It is present continuous I think.