What does "了" do in the sentence "呆在家里， 这样你就能接电话了"?
Judging from this webpage, it's a translation from the English:

Stay home so you can answer the phone.
呆在家里，这样你就能接电话了。

In this context, they've added the change of state 了 to refer to a future change of state: it's being said to someone who will change from

State 1: not being at home and therefore not being able to answer the phone, to
State 2: being at home and therefore being able to answer the phone.

Dropping the 了 would be appropriate if there were no change of state involved (e.g., the person is already at home).