Spotify already has 100 million Premium subscribers, and it's growing quickly. In the last three months, it added 8 million paying customers. It's been focusing more heavily on podcasts, and it recently added CarPlay support. Teaming up with AT&T, which has 77 million wireless subscribers of its own, could give Spotify another boost.

The partnership is similar to the collaboration between Verizon and Apple Music. Last summer, Verizon bundled a six-month Apple Music trial with its unlimited plans. It went well enough that Verizon now includes Apple Music in its top-tier unlimited plans.