As the holiday shopping season enters peak frenzy, Sprint's (NYSE: S) presence in Walmart stores around the nation is growing thin. But it's unclear why the relationship is on the rocks -- or perhaps ending altogether.

"Sprint's presence in Walmart has declined in the past month and is now de minimis," the market research firm Wave7 Research wrote two weeks ago. "Within the past month, Sprint has been replaced by T-Mobile postpaid in some Walmart stores, while Sprint has pulled out of others, with counter space now occupied by Straight Talk or going suddenly empty. The only two postpaid carriers now listed on Walmart.com are Verizon and AT&T."

Calls from FierceWireless to several Walmart stores across the nation confirmed Sprint's lack of presence in the chain's brick-and-mortar outlets. Staffers at every Walmart store Fierce reached said their outlet doesn't stock Sprint phones, and one confirmed that the store had previously sold the carrier's handsets.

Sprint declined to comment on the story prior to publication via a spokeswoman. A Walmart representative said Sprint's products are sold at roughly 500 of the chain's locations, which is a small fraction of the 4,177 outlets it operates in the U.S., 3,275 of which are "supercenters." A request to Walmart from Fierce for more information on the situation went unanswered.

Following publication of the story, David Tovar, Sprint's vice president of corporate communications, offered a statement via email. Tovar noted that Boost Mobile and Virgin Mobile prepaid handsets are sold at "thousands" of Walmart stores nationwide.

"With respect to our postpaid business," Tovar continued, "we are currently in 500 stores and are in active discussions with Walmart about expanding our relationship." Tovar declined to offer information regarding Sprint's waning presence in Walmart stores in the email exchange.

In a phone interview one day later, Tovar insisted the there's been no recent decrease in the number of Walmart stores selling Sprint's postpaid phones. "Spint's presence (regarding postpaid phones) in Walmart stores is unchanged," Tovar said. "It remains at historic levels. That's what I know to be the case."

Walmart continues to sell some Sprint prepaid phones through its website. The retailer also carries handsets from Boost Mobile and Virgin Mobile USA, both of which are Sprint subsidiaries. Wave7 observed that "Boost sales were ended at some Walmart stores several months ago," however.

The looming question is whether Walmart is squeezing Sprint out in favor of other service providers or Sprint has decided Walmart wasn't an optimal distribution channel for its brand. Both scenarios seem plausible: a carrier move to sacrifice shelf space in a high-traffic nationwide retail chain during the holidays might seem ill-advised, but Sprint substantially expanded its retail footprint earlier this year when it opened 1,435 Sprint/RadioShack stores.

And while Walmart is undeniably a retail mammoth, it's worth noting that it focuses far more heavily on prepaid phones than on postpaid handsets. In fact, Wave7 Research estimates that prepaid outsells postpaid devices "by far more than 10:1."

Nevertheless, Sprint is clearly losing shelf space in the world's largest retail chain at the height of the holiday season. Regardless of whose decision it was, the move could be costly to a carrier struggling mightily to add customers.

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Article updated Dec. 4 to include comments from Sprint.

Article updated again on Dec. 5 to include more comments from Sprint.