In the UK 2G signals are stronger and more available than 3G. If you need a stronger signal for voice and SMS switch off 3G and data to force your iPhone onto the 2G network.
On your iPhone hit Settings.
Turn off Enable 3G and Cellular Data.
Check your iPhones signal bars, have you now got an extra one? Upsides: makes your battery last longer. Downsides: you won't have data - so no emails or browsing