You may get an automated response thanking you for the report and giving you further instructions if needed. You will not be charged for sending texts to 7726.
An easy way to remember ‘7726' is that they are the numbers on your telephone keypad that spell out the word ‘SPAM'.
https://www.ofcom.org.uk/phones-telecoms-and-internet/advice-for-consumers/problems/tackling-nuisance-calls-
and-messages/spam-texts