This is all to do with the difference between past simple and past continuous or progressive:
in order to use the past continuous or progressive along with the past simple, you need to have two – not just one – simultaneous actions happening in the past (actions starting and finishing in the past, otherwise you would have to use a present perfect instead of a past) and they need to be of different lengths – if the lengths of the two actions are the same, you can get rid of the two past continuous tenses and replace them with two past simple tenses, as in
"John cooked supper while I watched TV."
"John was cooking supper while I was watching TV."
(Michael Swan's Practical English Usage, second edition, fourth impression, 1996, Oxford University Press, page 73, numbers 73.1 and 73.2)
The reason for this simplification, as usual, is "the law of the least effort": why write four words ('was cooking' and 'was watching' instead of just two, 'cooked' and 'watched') if two can make you understood as efficiently as four?!
This reasoning can be transferred to the present:
make the present continuous or progressive contrast with the present simple when there are two simultaneous actions in the present and you want to emphasise their different lengths (in your example sentence):
When you are talking on the phone, you can't see the other person.
Using 'are talking' instead of 'talk' – present continuous instead of present simple – emphasises the greater length of 'to talk to someone on the phone', and the shorter one of 'not to be able to see who you are talking to', meaning that time and again, and not all along, you might be tempted, while you are talking to someone on the phone, to try and see them and then realise that this is not possible.
This is not the 'usual' difference between present simple (for permanent habits, situations, states) and present continuous or progressive (for single actions happening at or around the moment of speaking, and temporary habits, situations, states).