I was informed by multiple people at T-Mobile that having a strong data connection doesn't necessarily mean a strong voice connection, but that confused me and I'm not sure if they're right. So how do the two relate? If there's strong LTE and weak EDGE would the iPhone show full bars or low bars, and would that tell you both how good the voice and data signal is or just one of those?
The bars on the phone are representation for Voice signals. A full bar will mean you will have the clear calls, on the other hand, the data can be seen by the type of Data service you are getting, LTE,3G(WCDMA),H or 3.5G(HSPA) and E(EDGE). Despite having the full connection bars you may have an E(For EDGE) which may give a slower network connection. It's the same thing the other way round as well, Despite getting a LTE network you may not be getting full bars, It depends on the Network tower you are connected to, the data it supports. Generally near the areas with people using less Internet over the network the companies put up towers which are good for voice signals.
Every phone only uses one antenna at a time, so, if you see the LTE icon on the statusbar, that means that it's connected on a LTE signal only, so, it goes for the voice and data signal.
tubedogg was right..EDGE is only a Data signal and doesn't support voice calls. same goes with LTE as well, momentarily LTE voice calls have not come into existence except few operators from US, UK & china. Apparently there are different signal strengths for Voice & Data signals and are clearly indicated on your latest smartphones. Voice signals & Data signals use circuit switching & Packet switching respectively and their transmission process are completely different.