That depends on what plan you are on. For instance, the plan my wife and I are on is $50 per month plus $9.99 per phone plus around $4.50 per phone in taxes and fees. If there was a 3rd phone on our account, the $50 charge would be spread over 3 users and therefore would be less per phone. If you are on a Share Everything plan, each phone has it's own fee that does not change, but you have a shared data plan. So, it depends.
on our bill it shows as each phone having its own charge. But the line leaving once the contract is up is the highest one. We have 2 smarphones and two reg phones. one of the smart phones would be the one possibly leaving.
Again, it depends on what plan you are on. If you are on a plan such as my wife and I, the smartphone leaving would eliminate their $9.99 charge and their $30 data plan, or whatever data plan they have. But, the cost of the base plan would not go down, therefore the cost per phone remaining would go up unless your reduced your minutes to a lower cost plan.
If you are on a Share Everything plan, the smartphone leaving would eliminate their $40 charge, but whatever data plan is on the account would still be there unless or until you reduce it to less GB. Also, the Share Everything includes unlimited voice and text. Any other plan, the text plan is $10 per phone or $30 unlimited for all phones. We need specifics if you want the correct answer.
It appears as if you are on one of the old Family Share plans and the phone leaving would be the primary line.
Being the primary line, this phone is charged the base rate for the plan plus $9.99/month for an access fee. Since it is a smartphone, there is also a data charge of probably $30 or more per month. When the line leaves, you will not have to pay the $9.99/month access fee or the data charge for the line leaving. The base account fee will continue to be charged.
When the phone leaves, one of the remaining lines will be designated the new primary line.