I've never done this with phone systems, but back in the cave man days when I was doing programming and modifications on accounting systems we used to have trouble with our customers being slow to pay their final bills.
Our solution was to deliver the system to the customer with our final invoice already entered into the accounts payable system, and we had code built into the software that looked at the status of the invoice to see if it was closed. If it was still open after the due date, for the first 30 days it would pop up a reminder on the screen once per day to remind them the invoice was past due, then for the next 15 it would warn them the software would revert to the "unregistered" mode, and if it still wasn't paid 45 days out it would place advertising on the bottom of every invoice they printed that said something along the lines of "This invoice was printed using an unregistered copy of Accounting Wizard by Company X. For more information on this software or to schedule a free demo please call our number".
Of course, one would assume we would have problems if we had a customer with a brain, so the code was set up to note the invoice date and number the first time the software was run by the customer. If they deleted the invoice it would immediately go into unregistered mode. If we had issues that were our own fault we could on in and edit the date as needed, and once the invoice was paid a flag was set to never check again so they could archive or delete the paid invoice at some point....
We felt these were legitimate measures we could take in this situation that would not damage the customer - just make them give us free advertising while we sought resolution of the problem in one way or another. Usually when the free advertising started coming out the check wasn't too far behind.
In the case of a PBX, unless leased or the initial purchase, this is a piece of hardware (not software) the customer buys, and it is their property. While software license rights typically state that you are not buying the software, but only licensing it, YOU are not the manufacturer and have no right to disable that software (in my humble, non-legal opinion). The hardware is owned by the customer, and anything you do to the hardware is akin to vandalism or hacking and you could probably get yourself in deep doo-doo (sort of like the System Administrator that gets fired and logs in with a back-door password and disables the ex-employer's systems). Now assuming this is the initial system purchase or a lease situation, then in that case I would assume (this is usually in your fine print on your purchase contract) that title does not transfer to the end user until the final payment has been made, or if they are in violation of the terms of their lease agreement I would assume that you could then remove the equipment that you own and are leasing to the customer. Again, I would assume these provisions are in the fine print of your lease agreement...
But then, you know what they say about assumption....
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