Nokia 6125 & Message Center.

I bought my Nokia 6125 brand new. Never branded or locked.

I am using T-Mobile.

At random times when I try to send a text message, it asks me to define the message center number. I go to the text messaging settings and go to the center number option. T-Mobile is there, I just have to set it as default again.

Then it works, then randomly it will do it again.

Any thoughts?