My iPhone always had a healthy 3-4 bars of signal strength inside my house and outside. In the last few months, I noticed a significant drop to 1 bar only inside and outside. I live in an urban setting and nothing physically has changed with my phone or house. When I travel to other parts of the city, the bar slightly increases to 2 sometimes 3. This use to be a well covered Verizon area. What has changed with the signal strength?
In all honesty, the bars on a cellphone are not an accurate representation of signal strength. They are an aesthetic representation of the signal strength. The signal strength is measured by decibels. Depending upon the make and the model of the phone, there can be quite a bit of fluctuation between the bars on one phone to another phone.
For example, if an algorithm of your phone says that for every 10 db you will see a bar and your friend's phone manufacturer has set it such that for every 15 db a bar will appear, their signal will always appear to be weaker even though they may be receiving more dbs (40 dbs which would be 2 bars as opposed to 30 which would show 3 bars). Bars could be compared to a gas gauge where they give you a general idea of what the signal strength is, but not the actual strength.
That said, besides the number of bars you see, what issues are you having using your service? Where are you located?