In some older learning material I came across, they use 「電話をかける」 for "to make a telephone call". When/why did this come to be replaced by 「電話する」 in popular usage?
It randomly occurred to me today that while these are indeed similar in meaning, they are not always interchangeable. If you're talking about making a call to someone/somewhere, either can be used:
事務所に電話する call the office
事務所に電話をかける call the office
But 電話する can also be a joint action. Consider the following:
彼氏と電話する talk on the phone with one's boyfriend
(Technically you could argue that the second sentence is grammatically correct, but the picture it gives is that of two people dialing the same phone together, so it doesn't match the meaning of the first.)
So while 電話する can be used in both a directional (calling to someone/somewhere) and a joint (talking on the phone with someone) sense, 電話をかける can only be used in a directional sense.
As a purely speculatory note, it may be that 電話する is more common today because it covers more cases than 電話をかける, which may lead you to think that 電話をかける is on the way out.