Can I transfer the number from a standalone Apple Watch line to a new phone?

A year ago, I bought my son an Apple Watch SE which we set up with Family Setup. It has its own 10$ a month standalone plan and phone number, but now I am planning on buying him a proper phone. However, when I select add a new line and put my son's number in, it says "Already a T-mobile customer, number can't be transferred to T-mobile. We'll provide a new phone number after your purchase complete." I'm confused because the number is already with T-Mobile so they should be able to transfer it to a new line, but this suggests that he has to get a new phone number, which would be a massive hassle because he would need to give all his friends a new number and would lose access to many online accounts that have the old number in it. Is the website bugged and I just have to go to a store to do this, or does he really have to get a new phone number?