Context: I am informing my contacts, such as friends and relatives about changing the number on WhatsApp.

I no longer use the old number. Please save it and contact me on this number hereafter.

Please check them.

Are "no longer" and "informing my contacts" suitable?

kumenglishContext: I am informing my contacts about my new number on WhatsApp.

I am no longer using my old number: 45737-38823. My new number is: 45876-08762.

Please save it and use it contact me.

kumenglishAre "no longer" and "informing my contacts" suitable?

Yes, those are correct expressions for that context.

'no longer' is another way of saying 'not ... anymore'. 'no longer' is more formal.

I no longer use that number.
I don't use that number anymore.