With the rise in smartphone usage that rely on 4g/3g signals as opposed to old basic handsets that used mainly 2g I see lots of posts saying since getting my new handset, the signal is worse than before. The text below may help explain the issue and give a solution.
At Present in the UK there are 6 frequency bands in use
o2/voda 900 2g/3g is not that greatly diminished by distance/ building materials, whereas 1800mhz EE / o2/voda and any 2100mhz 3g provider is. Also, remember that any 3g signal the cell size also gets smaller. All 4g on 800mhz is also not subject to the distance/building issues but 4g 2600mhz is, and more than the 2100mhz band and now the exclusive 2300mhz band high capacity poor in building
Mobile signals in the 900mhz o2/Vodafone etc 2g also penetrate buildings better unless the building materials have made the house a mini Faraday cage. The same for all 4g 800mhz providers
A real-life example
In the village I live in the pub that is some 3.42 miles (google earth measured) from the o2 mast direct line of sight the o2 900mhz 2g signal gets in through the 17th century solid stone walls (over a foot thick) 3 bars in most of the pub. Yet those on the now EE network have an 1800 MHz mast / 2100mhz 3g only 1.37 miles (google earth measured) from the pub yet struggle to get any signal in the pub having to put their phones on window sills etc or go outside to make a call send a sms etc.
This issue within building coverage these days is more to do with new builds/renovations being done to latest building regs with things like foil backed plaster board and foil insulation materials and high tech metal coatings on windows.
So for summary
o2 800 MHz 4g best at distance and building penetration
o2 900 MHz 2g/3g best at distance and building penetration
o2 1800 MHz 2g not as good as 900mhz (not too many masts in that band)
o2 2100 MHz 3g the worst for distance and building penetration (also the cell size varies by the number of handsets connected)
O2 2300 MHz 4g is the newest band and exclusive to O2 high capacity but poor in building
o2 has no 2600 mhz 4g (but would be worst for building penetration)
With most handsets defaulting now to the fastest connection o2 3g2100 /3g900 will be selected and gives the worst indoor coverage. Unless you are in a 4g 800mhz area when it will select that.
I always recommend when at home unless close to a 3g cell to turn 3g/utms off and go in to 2g/gsm mode only for a reliable call quality (assuming data is done via wifi) this also is advised if on a fringe 4g area and you see no signal on your device
I hope this helps explain the issue and helps people regain their coverage indoors they used to have. (2g)
Obviously, if you have always had a poor 2g coverage this will not help so looking at voip options may help if you have adsl and Wi-Fi O2 now have native Wi-Fi calling (handset dependant) and the o2ask app
Edit 8/1/2012 14.19 for accurate distances for example
Edit 23/04/2016 for updates for 4g
Edit 19/05/2018 for Wi-Fi calling and o2ask and new 4g band