In a different sentence, it used "אתקשר אל" instead of "ל"...is there a difference? They seemed to be in a similar context.
Yes, there is. With the unspecified form, אל would be used. For example: אני אתקשר אליו, but התקשרתי לשוטר.
You can also use אתקשר אל המשטרה here, but it is less common (probably because it's longer).
It's not incorrect, but I'll probably never use it.
I wonder why "אל תתקרבו" is not accepted. Any clue?
Should be accepted, if the rest of the sentence is OK.