If he is not in a EVDO coverage area then it would not show EVDO it would show CDMA 1xRTT.
EVDO stands for
Evolution-Data Optimized or
Evolution-Data only. Essentially it's a cellular network technology that provides your handset a faster internet connection. This is generally called 3G (3rd Generation). In order to recieve this you have to be in a area that supports it (the towers have to be upgraded basically).
Think of CDMA 1xRTT as the slower internet connection (as well as the connection that handles your voice / text - that's why it switches when you send a text).
CDMA (1x and 1xEVDO) do not support simultaneous data and voice. However if you are browsing the internet and a call comes through the device should disconnect and allow the call to ring through.
Call Guard is an option that basically makes it "harder" to make or receive calls while roaming.
As far as missing calls when browsing, if the DDTM option is unchecked then you should not have a problem. However it seems to depend on however the network feels that particular day. The vast majority of times it works fine for me (1x coverage area for me

) however if I am doing something intensive like navigation I might miss calls.