It might depend on what is supposed to be inside the file.
If you work in the telco industry, then you probably just have to use the text editor of your choice, as the file is just going to have text records describing phone calls.
Without more to go on as to where the file came from it might be a CorelDraw file. If so the you can open it with the following applications. You can try right clicking the file and check the details tab. It might tell you what created the file.
The basic problem with your question is that "opening" a file is very different from "processing" a file. You can "open" the vast majority of files with most basic text editors.
However, almost every question of this type in this forum actually intends to ask about how to "process" a given file. Or what programs can be used to "process" a file?
Unfortunately, the questions almost always refer to files by their extensions, e.g., "CDR". And the problem with that is that an extension is merely a possible hint. It's not guaranteed to indicate anything about what a file contains nor how to process it. You can assign almost any extension you wish to any file, and it can still be processed by the intended program.
Worse, a given extension might indicate multiple incompatible programs. One "CDR" file might be a simple text list of phone calls while a second one can contain a Corel Draw drawing. In order to know how to process one, we need to know how it was created or something about file details.
What can also help is telling where you got or found this file. Was it sent to you in an email? If so ask the sender what to use. If you found this on a device, what folder or directory was it in? That might help narrow down what the true CDR format of this file may be. File extensions can be any thing. I could create my text files or even word documents with a .BOB extension. As long as I set my system to open all .BOB files with Office, I'm all set, and nobody has a clue what a BOB file is.
Simply a CDR extension file is primarily supported by Corel Draw. Though other software/ tools can also open this type of file. If your system has Corel Draw it can be opened by double click on the file. If not, It asks for a software to open. If you have necessary software/ tools you can open this one else ask the source (from where did you get) for further help.
While more information from your side is always helpful.
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Simply a CDR extension file is primarily supported by Corel Draw. Though other software/ tools can also open this type of file. If your system has Corel Draw it can be opened by double click on the file. If not, It asks for a software to open. If you have necessary software/ tools you can open this one else ask the source (from where did you get) for further help.
While more information from your side is always helpful.