Can free line change number?

My plan has 10+ lines.  in 05/2024, I added a new line to my plan and got an additional free line.  This free line came with a random number.   Till today, no one use this free line.

In 08/2024, a friend joined my plan by porting in his AT&T number and T-Mobile charge $35/monthly for this line. 

I chatted with T-Mobile Customer Service,  and wanted to replace the free line with my friend number, so it can save $35 per monthly.  But the customer service told me that the existing free line number can't be replaced or changed.  IS THIS TRUE??  All I can do is get a SIM card with a random number doesn't make any sense to me.

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