Solution 1:

I suppose you are trying to match minutes 29 and 30.

why your expression does not work

[29-30] does not do what you want from it; it means

2 or (any character from the range from 9 to 3) or 0.

The range is indeed invalid because 3 comes before 9 (*).

how to do it

To achieve what you want (i.e. matching 29 and 30), you can simply use (29|30) in your regex, like

grep MESSAGE_SENT company-runtime-cdrs*.log | grep '27T20:\(29\|30\)'

or

egrep MESSAGE_SENT company-runtime-cdrs*.log | egrep '27T20:(29|30)'

(note that egrep needs less escaping so it is more readable)

your exact time range

I need to grep this time range: 2020-04-27T20:29:27 - 2020-04-27T20:3x (any time after 30)

this is a bit more complicated:

grep MESSAGE_SENT company-runtime-cdrs*.log | grep '27T20:\(29:\(2[7-9]\|[3-5].\)\|3.:\)'

or

egrep MESSAGE_SENT company-runtime-cdrs*.log | egrep '27T20:(29:(2[7-9]|[3-5].)|3.:..)'

further advice

I suggest to use a tool like regexr to test / explain your regexen. I used it too to check what I came up with above.

(*) formally this is locale-dependent, but I have yet to see a locale where it is not true