Hello there,
I have also recently ported from Celcom and been receiving spam messages like this. Please take note that it was send as MMS/ SMS. Even if you disable it, it will keep sending. And they label this "five digits sender"as 3rd party Company.
They have list of third party Company that charge the consumers however they will not block all this numbers for you unless you have been set as a victim (which mean you received SMS/ MMS and charged RM1.50) > You called Celcom > You complain the number > They will create report and block that
EXACT[U] number you reported.
I have already received from two different number aka two different Company. I have tried logging into OCS online with their online CS to complain yet no action is taken. I personally block that number in my phone so they won't be able to send you again in the future.
Please take note even if you reply "STOP ALL" it will additionally cost you 0.15 cents for the SMS, but it will not stop the 3rd party Company to send you as I have tried and received again from the same Company.
Using Iphone gave you a choice to block. Another way to avoid is not to open the SMS/ MMS at all. Because once you read it, that's where they start sending "Thank you for downloading InstaKlip - RM1.50".
The last time I received is from 39191 - JAVA. I call them and screw them for good 30 minutes and they keep telling us "You shouldn't open the SMS/ MMS". So I threw them back a question, "When we ported, you did not advise client that Celcom users are not suppose to use MMS/ SMS because there is a risk that you will be charge RM1.50 every time you accidentally "read"one!".
And they keep quiet. My final advise, keep paying until you are fedup, port out.

Actually I have called the CS to complaint this issue and asked them to block all MMS which charge a fee. Also threaten to lodge complaint to MCMC. Hope this work out well.