Past progressive: I was trying to call you at 6 last night. What were you doing?

[Giovanni]

Could you tell me whiich of the following I should use and why?

I was trying to call you at 6 last night. What were you doing?

or

I tried to call you at 6 last night. What were you doing?

Thant you for your help.

[Mister Micawber]

.
Because you have stated a precise time (6 pm), only the second is appropriate. The first could be used to stress the duration of possibly numerous attempts to establish telephonic contact; the second states the mere fact of that past action. For instance, you could use the first like this:

I was trying to call you all night last night, but you never answered.
.
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Native English teacher at Mister Micawber's

[Yankee]

Hi Giovanni

I agree with Mr. Micawber.

Let's rearrange your sentences and then look at what they mean:

Mary: What were you doing at 6 last night?
John: I was trying to call you.

In this case, the meaning is that John tried to call Mary at 6 but John's reply (past progressive) also suggests that there were multiple attempts to call Mary. In other words, one of John's attempts to call Mary happened at 6 pm and he also tried to call her at other times during the evening.
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"Nearly all men can stand adversity, but if you want to test a man's character, give him power." ~ Abraham Lincoln

[Alan]

I feel the exactness of the time (6 pm) tends to militate against a continuous tense. I would prefer: What were you doing at around 6 pm? But I have no desire to nitpick!

A
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English as a Second Language
You can read my ESL story Present Simple

[Yankee]

My comments support MM's view and provide an additional illustration of the sense/meaning when an exact time is mentioned (as is the case in the original sentences).

So, Giovanni, it appears you've now got three votes in favor of the simple past tense and three votes against the past progressive in your sentences.

Just an additional observation:

I'd say that it would be far more typical to ask the question "What were you doing at 6 pm?" than to ask "What were you doing at around 6 pm?" The use of the progressive tense indicates that the person is expecting the reply to be about an activity that was in progress at and around the time mentioned in the question and therefore the word 'around' is generally not added.

However, your sentence was not a question, Giovanni, and there was no context that indicated multiple attempts to make a call, so I would tend to agree with Alan that adding the word 'around' might help make the use of the past progressive more acceptable in your sentence: I was trying to call you at around 6 last night. Still, I'd be happier with more context that justifies using the past progressive in your sentence.
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"Nearly all men can stand adversity, but if you want to test a man's character, give him power." ~ Abraham Lincoln