It is actually out of courtesy.
Why most Chinese speakers will say "我接個電話", instead of "我去接電話" which syntactically is also correct?
Saying "我接個電話" actually implies that the disruptions of the current conversation by that phone call has an (expected reasonable) end (worth noting, it is not a promise or pledge to comeback to this conversation, rather it's a courtesy to imply I'm not leaving this conversation because I want to spend my time with something more fun).
The form "我去接電話" often sounds harsh in daily conversation, as it implies a "I'm abandoning you and go to talk to someone on the phone that I have no plan to end..." As a matter of fact, you might hear sometimes people will say it this way, to subtly imply that they do not enjoy the current conversation (i.e. argument between couples), and they'd rather go to talk to someone else on the phone, and not looking forward to come back to this conversation.
Think about this way, if you say "I'll answer phone calls", it sounds odd, right? "我去接電話" sounds somewhat similar to that.
First of all, we need to clarify, in Chinese language, there is no "count Nouns .vs. noncount Nounc" concept.
All nouns in Chinese language are noncount. To count anything, you will need to add the "quantity qualifier". i.e. a horse is called 壹匹馬, where the character 匹 is the "unit of measure for horse" (it can be used to measure textile too, but that's for another day).
So, back to 個. it is the most common unit of measure word. Majority of the English count nouns would be translated to using 個 as the quantity word. Which you mentioned you feel comfortable with "make a phone call" as 打個電話。
BUT, what's often missed, is actually the article "a" means "one", but if 個 is a unit of measure, that doesn't add up, right?
That's right. From a completely formal syntax perspective, "an apple" should be translated to "壹個苹果"， so "an" is translated to "壹個". It's just in common daily Chinese speaking, the character 壹 are often omitted.
When you say "I ate an apple", it is translated to "我吃了個苹果”， and if you say "I ate five apples", it is translated to "我吃了五個苹果“. As you can see, the character 個 did not disappear as the article "an" in the five apples case, and it is worth noting that 個 is not really a perfect mapping to the English article "a", instead, the better mapping from "a" to Chinese is actually the character "壹" in ""壹個", which is often omitted in spoken Chinese. This omission in Chinese, combined with the vast common usage of article in English made it harder to translate from one to another language with perfect precision.