When I was building the house, I put in a propane tank for a future generator (I don't have yet), Suburban was the only one who could provide the tank in the time frame needed.
They were going to charge me $300 for the extra length of copper they left (I returned & got that straightened out). They still are still holding my regulators hostage & have a lock on my tank. I own the buried tank.
Whenever I finally get my generator, it will probably be cheaper to buy new regulators than to have them come back & install the ones they are holding hostage.
I will deal with my local company when I need them......
If you OWN the buried tank, then they can't put a lock on it OR hold your regulators hostage IF you paid for both. And if that is the case, I think I would be getting rather firm with them in person, and get what is rightfully mine. As far as locking the tank.....Bolt cutters will take care of that.
Do you owe them money for the installation of the tank, or the labor to install it or run the lines? If not, visit them in person.
As far as my propane goes, I am on a fill at will, or also a call in. They will fill based on my usage from the prior year, or if I need it sooner, I will call them. I am locked in at $.999, or if I use more than 600 gallons, then I am locked in a $.949. Our previous supplier was charging $2.35/gallon. And the price went up from that, because his dad passed away, and he got into a legal battle with his two sisters. He owed them something like $100,000, so he passed that on to the customers. But he soon passed away himself, and the company got sold to the company we have now. I am very pleased with them. I do rent all of my tanks though. When I had the new tanks installed for the garage (2) 120 gallon tanks, the initial cost was something like $250, or $125/tank. That was to hook everything up. After that, I believe the cost is something like $1/tank/year lease. Which is way cheaper than buying tanks. Plus if you own your tanks, you have to have them checked after so many years. If mine start to look ****** with rust and such, due to weather, then they will replace the tanks as they see fit for nothing.
I also heard that if you have a buried tank, you have to have it checked after ten years, then every year afterwards for leaks.

And I heard from someone else that after ten years, you have to have the tank replaced. I don't know because I don't have a buried tank, but next time I talk to my supplier, if I remember, I'll ask them the particulars about a buried tank.