Four churches in the northern city of Monterrey, which lies some two hours by car south of the Texas border, are using equipment made by Israeli telecoms equipment firm Netline Communications Technologies to block incoming calls during services.

The Tel Aviv-based company was set up in 1998 by former military and defence industry specialists to develop mobile telephone jamming systems, mainly for the security industry.

"Before we had the system, it was very uncomfortable hearing calls coming in during the celebration of mass. But now it's 95 per cent quiet," said Bulmaro Carranza, a caretaker at the city's Baroque-style Sacred Heart church.

The signal-jamming equipment is packed into two wall-mounted boxes the size of small hi-fi speakers, with one beside the altar and the other at the church entrance.

Switched on just before the start of every service, the system causes a "no signal" message to be displayed on worshippers' phones, but causes them no lasting damage.

"We believe that we were the first church in Mexico to use this technology," Carranza said. "Now we are getting calls from all over the country to see how it can be installed."

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