Subscribers will get real-time alerts when they’re about to hit their monthly limit for voice, data, SMS and international roaming under voluntary guidelines announced today by the FCC, CTIA and Consumers Union.

The FCC has put its proposed bill shock regulations on hold as wireless operators work to adopt the new standards.

However, FCC Chairman Julius Genachowski said the agency would take a “trust but verify approach” and will monitor carriers to make sure the alerts are being provided as promised. The bill shock proceeding is being kept open in case operators don’t comply with the guidelines.

“This solution will give consumers the information they need to save money on their monthly wireless bills,” Genachowski said during a press conference. Unexpected charges can be particularly burdensome to consumers during difficult economic times, he said.

The policies will go into effect within 12 to 18 months. The free alerts will be automatic and won’t require customers to opt in.

CTIA’s board of directors approved the usage notification guidelines during a meeting at the trade group’s fall conference last week, said CTIA President and CEO Steve Largent.

Largent said the voluntary rules helped the industry avoid “excessive government regulations.”

About one in five subscribers have had unexpected charges on their wireless bill during the past year, according to estimates from Consumers Union. Some wireless operators such as AT&T, Sprint and U.S. Cellular already offer the alerts.

Consumers Union, publisher of Consumer Reports, will create a website with the FCC where subscribers can report any problems with the alert system.