Negotiations to complete the merger of T-Mobile US Inc. and Sprint Corp. are dragging on as the parties haggle over ownership restrictions and other conditions for Dish Network Corp. once it gets assets from the wireless companies, according to people familiar with the matter.

Discussions are continuing, and all sides remain optimistic they can find common ground on the ownership question and other issues to complete the more than $26 billion merger, the people said.

T-Mobile...