It can vary by phone, local terrain, and load, but generally, a -90dBm signal or greater (greater being -89 --> -50 or greater) is sufficient for phone use. Quality also comes into play and I know of no way of measuring that with phone apps. A wavering signal is suspicious if it fluctuates erratically a lot.
-Frank
I routinely make calls using signals a lot weaker than -90. -115 (give or take a few) seems to be the "drop" point, so a stable -95 will give you an acceptable, if a little low-quality, call. But here in the sticks, you take what you can get. -65 is considered a "strong" signal around here, -85 is routinely "normal", and if we didn't make calls beyond -90 we wouldn't be using our cell phones for much except really cool skipping stones!
But, yeah, fluctuations in a low signal can be a sign that your phone is struggling to talk to two towers, both of which are weak, or that you're suffering from some sort of irregular RF interference. Even so, as long as the signal isn't routinely dropping into the -105 or worse range, you'll probably be able to make a call. It won't be a fantastic call, but it's not likely it'll drop on you.
Sensorly and Opensignalmaps both have nice apps that allow you to examine signal strength, and "donate" that information to near-real-time (in Sensorly's case) and eventually-when-they-feel-like-it-time (in OpenSignalMaps' case) "heat maps" that show general signal strengths in your area. Where a Sensorly map is properly populated (see
4G + CDMA + GSM + WiFi - Coverage maps at your fingertips | Sensorly | Sensorly) you'll have a lot more fine-grained and accurate data than the carrier's coverage maps.
OpenSignalMaps has the better app since it also has a tower locator built-in (of questionable accuracy, but it's pretty cool and fairly accurate), but Sensorly has the better map since they actually use the data people submit and update their maps within about 15-30 minutes or so.
Running either app is a great education on signal strengths, while at the same time (especially in Sensorly's case) giving you information about what sort of signal strengths to expect where you are going.