The operators are Bharti Airtel, Idea Cellular, Loop, Reliance Communications and Vodafone.
"Telecom Regulatory Authority of India has initiated action by issuing show cause notices to service providers for violation of Mobile Number Portability (MNP) Regulations 2009 and the same will follow due process," said Milind Deora, Minister of State for Communications and IT, in a written reply to the Lok Sabha.
TRAI has received complains from some customers regarding wrong rejection of porting requests by telecom operators. The ministry said that it also came to know that customers have been charged Rs.10 for a message for requesting porting code through number 1900.
The mobile number portability service which allows customers to switch operators while retaining numbers was launched across the country on Jan 20.
TRAI had issued direction on Dec 2, 2010 to all Cellular Mobile Telephone Service Providers and Unified Access Service Providers prohibiting extra charges for messages sent to short code '1900'.
In order to reduce the porting rejections by the service providers, TRAI had also issued direction to the operators on May 24, 2011 specifying that rejection under contractual obligation can be done only if the post-paid connection comes with bundled handset with contractual obligation having exit clause and the subscriber has not complied with the same.
It also added the criteria of a corporate connection with contractual obligation having an exit clause and when the subscriber has not complied with the same.
Regarding outstanding payment it specified that if the due from the subscriber, in the previous paid bill, is less than Rs.10, then the service provider may include the same in the subsequent bill of the subscriber without any penal charges.