A strategic shift we made in our UX framework determined the parent grouping was per line, rather than selecting all plans first then moving to devices. To humanize the decision process, the first question we asked was "Who's line is this for?" to better frame the decisions that would come next (plan, phone, features, insurance, etc). This move also helped us on the account side allowing a clearer billing breakdown promoting the ability to easily split financial responsibilities.
A guided linear flow allowed focus and reduced cognitive load for our users. Each selection came with commonly asked questions that provided answers in a modal to reduce the need to bounce around to different pages keeping them focused on the task at hand and provide the information needed to make the current decision.
Context was also key and provided through a sticky progress element that also calculated money totals as you completed decisions for each line, allowing you to see a running total and how each decision affected the overall cost.