When I will be there, I will call you.
When I am there, I will call you.
When we will come back home, we’ll have a big party.
When we come back home, we’ll have a big party.
When when means ‘at the time that’ and refers to the future, use the present simple after it, not will: ‘When I get home, I’ll call you.’ We can use will in the other part of the sentence.
The same rule applies to future time clauses with as soon as, before, after and until: ‘I’ll call you as soon as I arrive.’