UKH

The phone signal in our flat has always been really poor, I guess we are sat in a concrete/steel box so it isn't surprising. Does anyone have any suggestions for some way of boosting it - maybe an antennae in the garden - we are on the ground floor?

Thanks for looking,

Chris

Not quite the answer you are looking for, but putting the phone on a windowsill can make a huge difference.

Speak to neighbours and find out which company has the best coverage in your area, also some handsets do seem to handle low quality signals better than others.

You can buy repeaters, but they are not especially cheap, it depends on how important it is to you really.

Change to a phone and network that enables Wifi calling. 

This works just as a normal phone call in and out, No extra app needed. The phone dose it in the background. 

A quick google: https://www.uswitch.com/mobiles/guides/what-is-wi-fi-calling/