In the fourth quarter of 2017, T-Mobile US Inc. wasn't only the top dog in the operating market, for which it won 50% of all postpaid net adds; it also dominated the wireless M&A market just as convincingly, buying 101 licenses, more than twice as many as either Verizon Wireless or AT&T Inc.

In fact, the next most active buyer pool was our "other" category, made up of a dozen different smaller carriers, which together bought 63 licenses, while Verizon and AT&T trailed with 49 and 41 license buys respectively.

What also stood out during the fourth quarter was the strong revival in activity. All in, 277 licenses were purchased (announced or closed). That three-month volume was well more than double the paltry 125 licenses sold in the prior six months (March-September). The latest quarter summary stats are shown below.

Click here for details on each of the license transactions recorded for the fourth-quarter 2017 period.

Large multimarket, multiband exchanges between T-Mobile and AT&T and T-Mobile and Verizon accounted for the vast majority of the 147 deals closed in the fourth quarter, which in turn were 53% of all fourth-quarter deals.

But headlining the 130 newly announced transactions were the first two post-Incentive Auction private purchases of 600 MHz spectrum, with AT&T the seller in both cases.

Private equity group LB Spectrum, backed by Columbia Capital LP funds, is buying 16 licenses in a dozen markets and TStar, an Incentive Auction DE winner, is buying the other seven licenses AT&T won in that auction. The two deals close out AT&T's 600MHz holdings.

Another notable fourth-quarter 2017 set of transactions occurred in the 700 MHz space among Grain Management, a minority-owned spectrum aggregator, and AT&T and Verizon.

In November 2017, AT&T filed to buy from Grain three 700-MHz licenses in North Carolina — Charlotte (CMA 61), Raleigh-Durham (CMA 71) and Greensboro (CMA 47) — which Grain had bought from Verizon back in 2013 as part of Verizon's sale, encouraged by the Federal Communications Commission, of all its 700 MHz B block spectrum won in the 2008 auction.

The 2013 sales were part of the quid pro quos related to Verizon getting FCC permission to merge with Vodafone Wireless. At the same time, Grain bought a big AWS-1 license in Dallas from AT&T. In both cases, Grain immediately entered into long-term transfer leases with AT&T (for the North Carolina properties) and Verizon (in Dallas), which have been operating the licenses since that time.

Now, after a five-year hold, Grain is selling the assets back to the two big carriers. There has been no release of financial terms. However, Grain bought the three North Carolina licenses from Verizon for an estimated $3.99 per MHz pop in 2013.

PCS license transactions dominated the quarter, with 145 licenses accounting for 52% of all buys and all the major players involved in the M&A. The rationalization of bands/blocks within the PCS and AWS license group has been a major driver of the swap deals playing out over the past two years. In the fourth quarter of 2017, AWS licenses accounted for 16% of the licenses sold.

AT&T was involved in most of the 25 700-MHz license sales in the quarter. It is still consolidating the few B and C block licenses in this band that it doesn't already control. And in the quarter it also traded four B blocks for C blocks from U.S. Cellular as the carriers aim to increase operating efficiencies.

In an addendum to an earlier filing, T-Mobile announced its planned buyout of the 46.4% minority partner in Iowa Wireless Network Services in the third quarter of 2017, then noted on its fourth-quarter investor call that it had paid $25 million for the equity plus an unknown amount of assumed debt for the minority stake, implying about $720 per customer, a decidedly minority discounted price.

IWS is a long-time operator in Iowa, eastern Nebraska and western Illinois with approximately 75,000 customers on a midband AWS/PCS network.

While pricing information for the vast majority of wireless spectrum transactions is not made public, T-Mobile did just share some value-related data in its 2017 10-K filing. The carrier noted that its spectrum transactions in 2017 totaled $8.60 billion. That, of course, included $8 billion for the 600 MHz licenses it bought in the Incentive Auction.

The remaining $590 million involved private M&A in three AWS/PCS spectrum exchange deals T-Mobile closed during the year. For those deals, it reported the value of the incoming spectrum at fair market value and its own gains on the assets it traded away.

Assuming that these spectrum swap transactions were primarily even exchanges of spectrum, we calculated a range of gains on the collection of spectrum licenses it has disposed of at 30% for a $123 million swap closed in March; 25% on another $115 million swap done in September; and 48% of a much larger $352 million even exchange of licenses closed in December.

The weighted average gain against T-Mobile's carrying value on the mix of AWS and PCS spectrum it sold was 40%. Of course, the holding periods for this large collection of licenses sold is varied and unknown, but it is of note that the underlying value of the company's spectrum continues to grow, even as more and more spectrum has been flooding into the market.

At the same time, T-Mobile shares over the past five years have risen 185%, its revenue 66% and its operating cash flow has more than doubled.

Wireless Investor is a regular feature from Kagan, a group within S&P Global Market Intelligence's TMT offering, providing exclusive research and commentary.