Short answer

When your T-bill matures, the units are automatically debited from your demat account, and the maturity amount is transferred to your primary bank account linked with Arrow. CDSL sends a message whenever shares, mutual funds, or G-sec holdings are debited from your demat account.

SMS format

The SMS will read: CDSL: DEBITED IN A/C *xxxxxxxx 100 TREASURE BILL OF GOVERNMENT OF INDIA TOWARDS REDEMPTION ON 18/01/2019. CONTACT YOUR DP FOR MORE INFORMATION.

You can view this transaction on your holdings statement.