iPhone aps are great – except when network gridlock prevents them from working. Coachella’s smartphone app was among those hit by poor AT&T service at the three-day festival in Indio, Calif. Judging from reports coming out of the festival, calling or sending texts to friends was often impossible. Hot geolocation apps like Foursquare, good for locating friends at large gatherings like music festivals, were often rendered useless.

The 2010 Coachella app, created for iPhones and Android devices, contained many features dependent upon a functioning network: an interactive map, a friend finder, photo uploading, and map marker that notes the location of a car or campsite. In theory, those are great perks for attendees. The friend finder is especially helpful at a sprawling event site with numerous concurrent events to splinter a group of friends.

In a statement to the SF Weekly, AT&T says it took measures to accommodate the additional traffic. Separately, the company was letting people know that Wi-Fi was available but it appears the word did not get out.

Indio was a “dead zone,” one person told the SF Weekly. Over the weekend, Twitter was full of criticism at AT&T for poor cellular coverage at the festival. One person spent six hours looking for a friend, “no thanks to AT&T.” Another wrote, “My lack of tweets and updates during Coachella brought to you by AT&T.” A tweet from music blog Stereogum claimed service was so bad AT&T should issue a refund to everyone who attended the concert. Musician Sondre Lerche, who did not perform, apologized to followers for not tweeting from the concert due in part to poor AT&T service.

Dependence on acceptable cellular coverage makes creating a custom music festival app a tricky proposition. C3 Presents’ Michael Feferman warned of this challenge in Billboard’s recent article about Foursquare. A promoter who builds an iPhone app, he explained, will face the complications of an overloaded network. When fans can’t use that app during the concert, a promoter’s investment will be negatively impacted.