Try these solutions;
1) Depending on what phone you have, you can use your home or work WiFi to seamlessly send texts and make calls - even without signal.
Setting Up O2 WiFi Calling
Information can be found on O2’s website
Setting Up Three Wi-Fi Calling
Information can be found on Three’s website
Setting Up Vodafone Wi-Fi Calling
Information can be found on Vodafone's website
2) Make sure your network/carrier settings are up to date. On iPhone, go to Settings > General > About. If an update is available, you’ll get an option to update your carrier settings. On Android, try updating the software in your Settings. You may also need to update your network/carrier version, either from your Settings app, or by going into your Phone app, then pressing the Menu icon followed by Settings. In there, you might find an option to update the version.
3) If you’ve tried Solution 2 above, and you can’t get signal anywhere, your SIM might be faulty. Try it in another phone on the same network, and if that device doesn’t receive any signal, please contact your network directly to order a replacement SIM and SIM swap.
O2: 0344 809 0202
Three: 0333 338 1001
Vodafone: 0333 304 0191
4)4) If you insert your SIM card into another phone on the same network, and it does get signal, your phone could be faulty. You might need to organise an exchange or repair, by clicking here.