Sonic is unable to reasonable anticipate what your cost will be for their fiber service. $40 dollars is the lowest price they can actually guarantee.
After that, there's voice-related taxes that vary not only month-to-month, but location to location. There is also an OPTIONAL router rental.
Do you actually expect them to post every conceivable scenario on a truck?
This is why the trucks generally read "From $40", I assume...
Actually, they can 100% guarantee that the price will be greater than $40. It is absolutely impossible under any circumstances
for anyone to have a bill lower than $56 and Sonic knows this because Sonic requires additional charges and fees--regardless of what your local taxes might be.
-100% guarantee that they will charge you their $6.50 "Federal Line Subscriber" fee. (And while the Fed allows them to charge this fee, this is a Sonic charge that Sonic is choosing to assess and keep out of their $40 advertised amount.)
-100% guarantee that they will require
you to rent their $9.50 router if you want the $10 temporary discount off the regular $50 price.
So while there are voice related taxes that vary by location, those are added on top of that minimum $56 a month charge. And while you are technically correct that customers have an option not to rent that $9.50 router, if they do so then they cannot get the $10 off and so then their minimum cost per month will be $56.50 instead of $56.00.
Bottom line: that $40/month number on the trucks is a lie 100% of the time for 100% of Sonic customers.