AT&T and T-Mobile might want to think about investing in some carabiners and crampons, because an FCC official just shoved a mountain in front of their proposed $39 billion merger.

The FCC, or Federal Communications Commission, regulates non-federal use of radio, television, satellite, and cable communications, and would have to either approve the AT&T / T-Mobile deal, or flunk it. Early reactions to the acquisition announcement intimated the merger was a foregone conclusion. Not so fast, says the FCC.

“There’s no way the chairman’s office rubber-stamps this transaction,” an FCC official told the Wall Street Journal. “It will be a steep climb to say the least.” Off the record, however, the official said the FCC hadn’t launched its formal evaluation and would vet the deal on its merits as well as whether it serves the public interest.

If AT&T’s purchase of T-Mobile USA goes through, it would make the telecom giant the largest mobile operator in the U.S. We’re talking nearly 130 million subscribers, and a total lock on stateside GSM network technology (GSM accounts for roughly four-fifths of total global market use).

What’s next? We wait, probably a year or more, for the FCC and U.S. Department of Justice to make up their minds. Plenty of time, in other words, for T-Mobile customers to either celebrate the deal, shrug, or share their annoyance.

More on TIME.com:

So Will the AT&T and T-Mobile Deal Get Approved?

What the AT&T & T-Mobile Acquisition Means for Everyone

AT&T & T-Mobile: The Early Reactions