ICCID/Phone number on SIM: Unknown

[_0x]

I have an old account online in which I need to prove my mobile number belongs to me.

The instructuons are: "Go to Settings > About device > Status. The phone number will be listed under ICCID."

But my ICCID/Phone number is "Unknown".

How can I get my phone number written to my SIM so this no longer shows as Unknown?

Thanks!

[MI5 · Accepted solution]

@_0x 

You need to put your SIM in an iPhone or an old Nokia as these are the only phones that allow you to write to the SIM. 

You then go into phone settings and enter your number and save it. 

It will then be on the SIM card and show the number in other phones.