But will your phone work in the desert?

Now that it’s officially summer, Californians are running for the hills, mountains, deserts and oceans for a bit of vacation time. But if you’re one of those people for whom vacation means obsessively checking Gmail on your BlackBerry, the time away can be maddening.

That cottage in the mountains is charming enough until Day 3 without cellphone reception.

So we did some homework for those of you considering excursions into the boonies this summer. We compared the coverage maps of the big four cellphone providers (available on their websites) with the boundaries of five of California’s most popular national parks: Joshua Tree, Death Valley, Sequoia, King’s Canyon and Yosemite.

Having cellphone coverage could be very helpful when you’re lost or wondering whether you really can eat those berries.

Turns out that wireless coverage is spotty in most of California’s national parks, but not everyone is similarly afflicted. Climb Yosemite’s Half Dome and pull out your BlackBerry to call your mom, and your phone might not work -- even though the hipster in hiking boots next to you is chatting happily on his iPhone.

Our conclusion: If you’re buying a mobile phone just for the reception in the wilderness, go for Verizon Wireless and stay away from T-Mobile. Although your best bet is probably to get a homing pigeon.

* In Yosemite National Park, where hikers can gaze at waterfalls and climb large blocks of granite, T-Mobile’s map shows that it has no reception anywhere, not even once you leave the park and are merely in the boonies. Verizon’s map shows it has roaming coverage in Yosemite Valley, as do AT&T; and Sprint. Rick Deutsch, author of “One Best Hike: Yosemite’s Half Dome,” said that making calls from the top of the granite rock has become common practice and that most carriers seem to get reception.

* Moving south to Sequoia and King’s Canyon national parks, where tree huggers can hug really, really big trees and climb the Sierra, AT&T;, T-Mobile and Sprint all lack reception. Verizon’s helpful map, which has shaded green areas to tell you where the national parks are, shows that customers will have coverage in the southwest parts.

* Death Valley, just east of Sequoia and King’s Canyon, isn’t a place many sane people would go in the summer, even though the sand dunes and mountains are pretty amazing. But if you are going, don’t depend on your cellphone when your car breaks down in the middle of the desert because you had the air conditioning jacked too high. None of the wireless providers offers much reception to speak of. Sprint’s map shows a bit of roaming coverage in the west part of the park, although the map is a bit difficult to read (it doesn’t show many towns to use as reference points).

* Finally, there’s Joshua Tree, the park of choice for Southern California rock climbers and cactus lovers. This is your best bet out of all five parks for getting cell reception on a hike. AT&T;, Sprint and Verizon all show moderate service, mostly in the northwest corner near Twentynine Palms, whereas T-Mobile’s coverage is on the east end of the park.

Why such discrepancies? It’s all about a company’s infrastructure and shared networks. Verizon, for instance, has worked out roaming agreements with smaller carriers, operating its service on their networks and charging customers a roaming fee. That’s why Verizon customers in Yosemite get some service but T-Mobile customers don’t.

A T-Mobile spokesman said that the company hadn’t gotten many requests for service in remote parks such as Yosemite and Death Valley, and that T-Mobile prefers to set up its own cell sites rather than negotiate roaming agreements, which can prove quite costly. That may change if T-Mobile customers start to request coverage in the parks.

Casey Schreiner, an Angeleno who writes Modern Hiker, a blog about hiking in Southern California, said that even areas close to the city, such as the Angeles National Forest, get spotty reception -- although he was once atop Mt. Lawlor without a signal on his T-Mobile phone while a friend with AT&T; was able to call his mom.

Hikers might have a better shot at reception in the Santa Monica Mountains along the coast, he said, although “you shouldn’t depend on a cellphone as your primary emergency signal.” Hikers worried about losing touch in the wild should instead buy a GPS satellite beacon, he said, which can emit an alert signal if you get lost.

You could argue, of course, that people hungering for cell reception even in the great outdoors deserve to go through some withdrawal. But many customers, especially those with vacation homes in the wilderness, don’t see it that way.

“If someone takes a vacation in the same spot every year, we hear about it,” Verizon spokesman Ken Muche said. “They’ll say, ‘You have coverage in Yosemite and so-and-so doesn’t, and we’re really grateful for it.’ ”

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alana.semuels@latimes.com