smart phone basic phone plan change


i want to upgrade my smartphone and upgrade the plan.

the problem i am running into is this

i have a smartphone and a basic phone on my plan. i choose a new smartphone as an upgrade and choose a new plan for the upgraded smartphone. 

now i am prompted to choose a new plan for the basic phone (which uses zero data) and am only choices i am given is plans with unlimited data starting at $70 plus $20 line access. 

so how do i upgrade my smartphone and plan and still keep the basic phone on the $30 basic phone plan (voice and text only)?

have called customer service and we just went around in circles and we mutually agreed to call it a day:)

thanks for any help or solutions