How to evaluate a phone data vendor before you pay
Four checks that separate a maintained catalog from a resold import, in the order you should run them.
Start with the preview
The preview is the cheapest test you will ever run, so run it first. Look at whether the fields are the ones you need, whether the identifiers arrive masked, and whether the file format is what your tooling can read.
A preview that shows a formatted row in the real delivery shape is worth far more than a screenshot of a dashboard. You are checking whether you can consume the output, not whether the design is attractive.
Ask for the delivery contract
Before money changes hands, you should know exactly what you will receive: the column list, the encoding, the file structure, and whether a manifest travels with the data describing how the set was produced.
A manifest is the difference between a file you can audit later and a file you have to take on faith. If the vendor cannot describe how a delivered set was generated and counted, the row count is not verifiable either.
Check how the price is computed
Pricing should follow the rows that will actually be delivered. When you change a filter and the estimate does not move, the number on the page is a package price rather than a quote, and the delivered quantity is a separate question you would rather resolve before paying.
A reproducible quote is one you can regenerate: same filters, same quantity, same figure. If re-running the same selection gives a different amount with no explanation, ask what changed.
Plan your lawful basis before you buy
Decide why you are allowed to hold and use the data before you license it, not after. That reasoning belongs to your organisation and depends on your jurisdiction and your purpose; a vendor can tell you how a file was assembled, but it cannot supply your lawful basis for using it.
If the intended use is not clearly permitted where you operate, treat that as a stop sign rather than a detail to sort out later. Consult qualified counsel for your jurisdiction instead of relying on a vendor's summary of the rules.