|In general, coverage in Millom and South Copeland is not good. There is significantly poor reception up in the hills and, while there are improvements around lower-lying land, break-up while moving is common as are difficulties with getting signals in buildings. Patterns aren't obvious between providers - some work well where others don't and reliability is a frequent problem. Being stationary while using a mobile phone is the best option.
Mobile phone service providers have maps of coverage on their websites - but these too should be taken with a large pinch of salt as they don't go to the depth of detail to accurately reflect the situation in reality. They also don't take into consideration weather conditions which can mean where you had a signal yesterday, you don't today.
While the advice is 'trial and error', you will find where's good for you, the signal and your particular phone with experience and from this will be able to use the phone with reasonable reliability.
However, it cannot be stressed enough that you can't assume your phone will work in an emergency situation while climbing the Lakeland Fells. It is very important that you properly equip yourselves with suitable clothing, especially footwear, map and compass, water, food rations and first aid supplies, including any regular medication - just in case. It must also be pointed out that phone map apps are generally not good enough to be suitable. Also, before you go, make sure someone else knows your intended route as well as a rough idea of when you plan to return.
UK Mobile Coverage has lots of useful information on mobile phone services including coverage maps for all the leading mobile phone service providers. Click on your provider's logo on that site to view their coverage map. Please remember, these maps are not sufficiently detailed to give accurate indications of signal strength. Alongside some of the maps is more coverage information as a generalisation. This information is useful guidance for various mobile phone facilities.