By law, you can use the device to use with any videorelay service. You are not limited to using only Sorensen But since you already made the plans to change the device, it's moot anyway (though, you can still use the ZVP with any VRS).
As for the flashing system, I looked up Z20 and I see it is part phone. So, look up Harris Communications or any other deaf products site and look at the
phone accessories. I have personally used something
like this one for my phone and door. If the costs are a problem, see if your state has an organization for DHH people; they might have grants to cover all or part of the costs for you.
A question: I see that the Z20 is part phone, so it is hooked up on a landline or the internet line?