NEW YORK — The company behind the BlackBerry smart phones said a three-hour outage Monday was caused by an upgrade designed to increase capacity.

Research in Motion Ltd. yesterday said the upgrade was part of "routine and ongoing efforts," and that similar upgrades in the past had caused no problems.

The outage, which started about 3:30 p.m. EST, annoyed subscribers who are used to checking and writing e-mail whenever they're in cellular coverage and able to make voice calls. It affected only some of the BlackBerry users in North America — for others, the service kept working fully.

It was the second major outage for the service in less than a year. In April, a minor software upgrade crashed the system for all users. A smaller disruption in September also was caused by a software glitch.

RIM, which is based in Waterloo, Ontario, has been on a tear, adding 1.65 million subscriber accounts in the quarter that ended Dec. 1, for a total of 12 million subscribers worldwide.

Experts said RIM's system is relatively reliable, but its centralized structure means that when there are problems, they can affect millions of users.

Blackberry e-mails in North America all go through a Network Operations Center. It appears the problem occurred there, when one of two Internet addresses that relay e-mail from corporate servers stopped responding, according to Zenprise, a Fremont, Calif., company that helps firms troubleshoot BlackBerry problems.