if you change your signal circles to actual numbers (key in *3001#12345#*, to get out, just press the home button), you'll find that there is a drop of about 20 by upgrading 7.1 to 7.1.1, or should I say downgrading for all iphone 4s users. So your phone is working much harder to get the same signal strength as before, and some of us will just drop calls.
Signal strength on mobile phones are roughly -113 dBm (weak) to -51 dBm (strong), at -80 (considered good signal strenth) and the signal circles will show full strength (5 filled circles).
Mine used to be at -69 dBm on 7.1, now after "upgrading" to 7.1.1 is showing -89 (much weaker), and my batteries are draining much faster. I don't think is a network problem, coz my friend never upgraded and still showing -69!
It will be unlikely that Apple will consider fixing this, unless they value their loyal customers as much as they used to, my guess is the program is now geared to favour 4G instead of 3G.