Many treatment quotes start with a phone call. You describe the house, the water complaint, and maybe a lab sheet, and within a few minutes someone names a system and a figure. That number can give you a rough sense of the range, but it is not yet something you can compare with a second quote unless you know what it rests on.
Ask what was measured, not what was assumed
Ask the provider to list the inputs behind the number. For a private well, that usually means the water test results you supplied, the flow and pressure readings at the house, and the household size they used. For a city connection, it means the source water report and any pressure or hardness figures they relied on. If the answer is that they estimated based on a typical home, treat the quote as a guess and write that down beside it.
Compare the phone promise with the site visit
Once an in-person visit happens, compare what was said on the phone with what the technician actually found. Ask whether the final price will change if the install spot, the pipe run, or the power supply differs from what you described. Ask whether they will measure the wall or floor space where the unit will sit, the distance to the pressure tank or meter, and whether there is a nearby outlet. A visit that checks only the water and never looks at where the equipment will go is incomplete.
Put the assumptions on both quotes side by side
Line up the inputs from each offer. If one provider assumed a household of two and another assumed five, the difference in capacity is not a difference in quality. If one used the hardness figure from your lab report and the other used a figure from a neighbor's test, you are not comparing the same job. Note each assumption in one line beside its quote. When the lines do not match, ask the provider to redo the quote on the same numbers before you judge price or equipment.
Ask what would change the total
Ask each provider which findings would move the total up or down. Common examples include a water test that shows a contaminant missing from the first sample, a pump that needs attention before a new unit can run, or a pressure reading that points to a different setup. A careful quote names those conditions in writing. A quote that says nothing about them has either not considered them or plans to raise the price on the final invoice.
Use the phone estimate for what it is good for
A phone estimate can still help. It can show you whether a company asks sensible questions about your water and household, and whether it is willing to measure before it prices. Treat the number as a starting point for scheduling a visit, not as something to sign on. If you are building a shortlist, the directory of water treatment providers can help you find companies to call, and the methodology page explains how providers are reviewed.