Can I keep my prepaid number with online activation of new phone?

I currently have a prepaid plan with t-mobile. I'd like to buy a new phone and sign up for the 30$plan(100 minute/ unlimited web, message) and keep my old phone number.

I've read that the 30$plan(100 minute/ unlimited web, message) is only for on-line activation. Can I keep my old number with the online activation in this plan?

I'm in the US.

Thanks.