The difference between the 2 telephones ...

Din that would depend on how many diffs

The most appropriate, easiest and most correct way to express this sentence:
"The difference between the 2 telephones cannot be noticed."

There's no need to include the words is or are.

However, that being said, if there's only a single difference, then it's a singular application, so you'd use the word is.

If there are more than one difference and your sentence use the word differences then it's plural, so you'd use the word are.

I think you've got this the wrong way round. If there is no obvious difference between the two, why start with "the difference"?

I'd say "There is no obvious difference between the two telephones".

or maybe "The two telephones are virtually identical".

If you are using "difference," then you would us "is." There is no way you would ever use "cannot" in such a sentence. Where the telephones are made is completely irrelevant to the question of English grammar.

The difference between the 2 telephones are/is cannot be noticed.
What is the subject on the sentence?
I think '2 telephones' is the subject.
If I am right, we must write 'are'.

What cannot be noticed, the difference or the 2 telephones?

Difference is the subject. The meaning remains the same if your remove "between the 2 telephones".

As Ragman pointed out, the sentence is wrong either way.

How do you feel reading this:

The difference between the 2 telephones is cannot be noticed.
The difference between the 2 telephones are cannot be noticed.

That is absolutely wrong. "The difference" is the subject of the sentence. Is English perhaps not your native language?

You are barking up the wrong tree, in more than one way.

I refer you to my previous helpful answer. Do not concentrate on "the difference" if there is no noticeable difference.