Hello. I also faced this problem and already found how to get rid of it. I can recommend you ask UCtel for help and use their o2 phone signal booster https://www.uctel.co.uk/signal-boosters/o2-signal-booster . It is legally available and really helps to boost the signal where it is weak. In addition, thanks to them, I was able to forget about problems such as distorted voice transmission and much more.