How to add back 2nd line to 55+

Stupid me, I cancelled my 2nd line on my unlimited 55+ plan. I currently have just one line, now, even though I'm paying for 2 lines, as my bill never reduced when I cancelled my 2nd line. When I use the online interface to add the same phone back on my account, it tells me I'll get an additional $88 charge.

How do I add the same phone I used to have as my 2nd line back to my service, without adding $88 monthly to my bill for my 2nd (used to be included) line? Gracias