The link to the APX file had been offered when the App was in Beta, but I haven't seen it for a while. There are sometimes ways to get Google Play onto some tablets that don't normally offer it, I've done it with a couple. Each used different methods and doing so runs the risk of "bricking" the tablet, so you take your chances. You might want to do some searches to see if it is possible with yours, just understand the risks involved.
Other than that, or 3CX feeling generous and making a download link available, you might be out of luck.