iPhone Field Test mode permanently in iOS 7 I have seen 2 different methods of putting your iPhone in Field Test mode permanently in iOS 7, a short one and a long one. 

The short way:


Open your phone app and dial *3001#12345#* (I recommend putting this in your Notepad so can copy and paste it. You will need it several times.)
Press and hold the power button until the slide to close appears. 
Hold the home to kill the Field Test mode app (In iOS 7 it will restart, in iOS 6 it is now permanent.)
Repeat step 2 and 3 a couple of times. 
Tap the home button to close Field Test mode.
Repeat steps 1–3 (With a little bit of luck it will work the first time here. I am usually able to get it the first time here. If not keep repeating.)
When it works you will see the Field Test mode actually close and NOT restart. When this happens it is now permanent. 


The long way:


Open your phone app and dial *3001#12345#*.
Press and hold the power button until the slide to close appears.
Slide the slide bar to turn your phone off. 
Turn your phone back on.
It should now be permanent. 


Sources:


http://forums.imore.com/guides-how-articles/271233-guide-signal-strength-changing-dots-decibels.html
http://forums.imore.com/iphone-5s/263849-numbers-instead-signal-dots.html#post2092268


Has anyone found any other ways to put your phone in Field Test mode permanently with iOS 7?
For iphone6 and ios8.1.1 once I was in field test mode I had to hold down power and home buttons - hard reset the phone to get the #'s to stay - all other methods the bars/dots came back