Philippine lawmakers have been left red-faced after an ID card bearing a photo of a "monkey" was used to breach new SIM card registration rules aimed at combatting rampant text messaging scams.

Under the law signed by President Ferdinand Marcos in October 2022, mobile phone users are required to provide a photo and other personal details when buying a new SIM card.

The rules also applied to tens of millions of existing users, who risked being disconnected if they did not register by the July 25 deadline.

But, rather than stop spam and scam text messages, the telecom regulator told a Senate hearing this week that there had been a "sharp increase".

To demonstrate how easy it was to get past telecom providers' automated vetting systems, a video showing a police officer using an ID card with a picture of a grinning "monkey" to successfully activate various SIM cards was played to exasperated senators.

"You have a terrible system if you can see a monkey and yet you approve" the application, Senator Joel Villanueva told the hearing on Tuesday.

Senator Grace Poe, one of the SIM card law's principal authors, lamented how text scams continued to defraud unsuspecting mobile phone users by offering fake jobs, lottery winnings, loans, and "even fake love at times".

National Telecommunications Commission chief Ella Lopez said there were more than 118 million registered post-paid and pre-paid SIM cards in the Philippines.

After the July deadline there was a brief drop in complaints filed to the regulator by scammed mobile users, Lopez said.

Since then, however, there had been a "sharp increase", with more than 45,000 reports lodged, she said.

To buy a SIM card in the Philippines, people are allowed to use one of several government-issued ID cards that do not have their biometrics.

Jeremy Lotoc, cybercrime division chief of the National Bureau of Investigation, told senators that fraudsters, including online gaming operators, were hoovering up cheap SIM cards bought from unofficial sellers at 40 pesos (71 cents) each.

But he said it was difficult for law enforcement to catch them.

"The issue is, once you use the SIM (for a crime) and get your objective, it is discarded. So it is very difficult to find it," Lotoc said.