Do you think girls should talk to their in-laws before the wedding, i.e. on the phone etc. when the her parents are not there.
Well If they call in the absence of the parents…she can’t say No to talking. I think if it was me…all I would manage would be a ‘ji’, ‘ji nahi’, ‘theek’, ‘hmmm’ 
Particularly if the in-laws initiate the conversation.
In-laws should be the one to initiate convo…warna tau ajeeb lageyga i guess lol.
Or is it better only to speak to them when parents are there.
It is better yes.
Also, what about them calling, & sometimes parents telling you to respond to them and then sometimes not to, is it better just to go with as they say, although then it makes it awkward when they call & you kind of can’t ask your parents right there and then?
When they call…You should talk…either when you’re asked to…or you happen to pick the phone.