From the press release (pdf) for the Interbank Mobile Payment Service launched by the RBI and a number of Indian banks:
The service provides an inter operable infrastructure for the banks to offer real time money transfer facility to their customers through the mobile channel. Banks are free to use any mobile banking application of their choice. Since IMPS can be made available in all forms (SMS, USSD, thin Client, Thick client), it can support the transactions from low end mobiles to high end mobiles to serve everyone’s needs.
The payer may use either SMS or a bank-provided mobile app; the payee doesn’t need any of these. The ‘participating banks’ are ICICI, SBI, HDFC, Axis Bank, Yes Bank, Union Bank of India, Bank of India.
From what I can gather Union Bank of India has instructions on both SMS-based and application-based transfer. SBI, ICICI and Axis Bank require a mobile app to make payments. Yes Bank has a generic FAQ (pdf) that refers vaguely to a ‘mobile application’. And there appears to be no information for HDFC’s and Bank of India’s IMPS at all.