Thanks for your post, sorry to hear you are having trouble with your mobile signal.
If you are happy to share your post code with us on here, I will be more than happy to check this out for you. If you aren't happy to share your post code, I can take the general area you are in, however, the results may not be as accurate.
It's no good getting a refund if the signals are still out.
Perhaps the only way to get any satisfaction is to change providers. I have had to make some highly sensitive calls due to family illness so I need a system that is 100%.
To be fair no network is ever going to be 100%
Although it does sound like you may need to look at alternatives to a purely mobile service. Diverting your calls to other mobiles or landlines, WiFi or app-based calling if you have good home internet, or other equally imaginative methods.