You missed the point. In my home, I have one landline and four handsets. I only pay for one landline, and I do not pay a per-handset fee.
I have one broadband connection and too many Ethernet and Wi-Fi devices to list. I only pay for one broadband connection, and I do not pay a per-connection fee.
I have two mobile plans and two mobile phones. I pay for each plan and phone individually. I should only have to pay for one plan. I'll concede an additional charge for additional phone numbers.
Slightly different there, as with your one landline and 4 phones, you might have 4 phones but you can only use one line at a time, thereby only using one line's worth of the company's resources. If you have two cell phones, you can use them simultaneously, using twice the company's resources rather than just enough for one phone. We won't go into broadband sharing or call waiting or three way calling. There are "family plans" with shared minutes if that's the kind of deal you're looking for, but it doesn't sound fair at all to have two different active cellphones and pay for only one.