Do you mean the number porting security code text message?
You need your old provider's SIM installed and working in your phone in order to receive that.
See here:
Once you have confirmed that your handset is unlocked, pop your old network’s SIM back into your phone, and we will send you a security code via SMS in order to move your number.
If your old network SIM has no service or you can't receive the security code SMS, then you will need to make contact with your old provider as they will have visibility of your old account.
If you are referring to some other text message/security code then please clarify the details.
If you need to contact 48 support about this then see the links in my signature below.
