3603399740

I have att as well however when I call the number says cant be completed as dialed try again later and its a different number then when I bought my phone

I have the same  number in my text message center what's the purpose & will it make something malfunction in my phone if I remove it?

The number is what your phone uses for sending/receiving messages. The message service center is the portion of a mobile phone network that handles text message operations. It is responsible for receiving, storing, routing and forwarding SMS messages from mobile handsets.
If you remove it, you won't be able to send/receive text messages.
You can change the number if you don't like this one, if you know a better number with better service. Inputting a wrong number will prevent your phone from sending/receiving any texts.
https://www.att.com/device-support/article/wireless/KB410175/ATT/Z221

I noticed the same thing. It's why I'm here

If you actually came here for answers then why did you stop reading at that post?