Why can't they swap plans on two numbers on the same account?

The Mobility team was less than helpful.  They said not only could they not do it, they could only offer to switch some plans around such that I would end up paying more but getting only the same or worse services.  Then they put me through to "Loyalty", who made me a slightly better offer, but I couldn't waste more time on the call - and asked them to send me an email with the details.  But they didn't.  They sent me an email with a teaser of info and said call back.

When I did, Mobility told me Loyalty was closed for the day.  Then they said they would transfer me to another group that could help.  Which turned out to be Loyalty, where I just got the message they were closed for the day.  Customer Service is awful.  Reaching out to Entwistle now.