Spam calls and text

[Aliebongo]

I received 2 calls and a text messages from 07748 808731

this is the message I received 

"You missed a call from me at 15:58 05 Nov.This is a free Call Alert from O2. To disable this dial 901, press *, then option 5 and option 5."

This was spam.  
DO NOT RESPOND, FOLLOW INSTRUCTIONS OR ANSWER THE CALL

[jonsie]

It's just that you have call alert enabled rather than voicemail

Just dial 1750 to turn on voicemail

Guide: All you need to know about O2 Voicemail 901