I had a quick look at the coverage map on the link below and I cant see any obvious issues. Did this just start happening or has it been like that from day one?
Beyond that common questions are:
1. What phone?
2. When does it happen?
3. Is it specific to one location or multiple locations?
4. Have you put your SIM in another phone (just to rule out your phone as potentially faulty?)?
5. Check the status that @Yeldarb suggested too, and that page lets you log a fault too!
Are you on WiFi when the call cuts out?
The cut off happened again about 30 minutes ago. I was talking to my son when suddenly he couldn't hear me.Each time I was sitting about 4 feet from my computer wifi . I actually went to the site you mentioned & filled in a form complaint which I am not sure reached anybody because the security capcha was playing up something awful. I redid the capcha bit as asked 4 times & the submit button didn't even register.
Was your phone on WiFi when the call dropped out?
What phone are you using?
Because I'm wondering if the drop out is due to WiFi Calling being used, and it's not able to properly fall back to 3G/4G correctly so the call drops out instead.