Why a phone cannot do this
Two separate walls. The first is the group thread, which becomes unusable long before 500 people and shows every number to everyone.
The second is the carrier. Ordinary mobile numbers are not built for bulk sending, and volume from a personal number gets throttled, filtered, and eventually blocked. It looks like spam because the pattern is identical to spam.
Sending individually is theoretically possible and practically absurd. Five hundred messages typed by hand is a day of work and the carrier will stop you long before you finish.
What makes a large send actually arrive
Business messaging runs through registered numbers under a system US carriers introduced called 10DLC, which ties a sending number to a known organisation.
That registration is what buys deliverability. Carriers treat traffic from a registered number as legitimate business messaging rather than as an unknown phone suddenly sending hundreds of texts.
This is handled for you rather than being something you set up, but it is the reason a service can do what your phone cannot.
What it costs at each size
Nineteen dollars covers up to 500 recipients, seventy nine covers 2,000, and one hundred and ninety nine covers 5,000. Each is a single payment for that list.
At 500 that works out under four cents a recipient, and it falls at the larger sizes. There is no monthly fee underneath it and nothing to cancel afterwards.
For a comparison against subscription platforms, which price by monthly list size instead, the competitor comparison sets them side by side.
Building a list that large
You cannot upload one, and that is deliberate. Recipients join through a sign-up link or QR code, so every person on a 500-person list chose to be on it.
That takes longer than importing a spreadsheet, and it is the reason the messages arrive rather than being filtered. Lists built from purchased or scraped numbers are what get sending numbers blocked.
For a large list the sign-up link usually goes into several places at once: a website, an email footer, a printed flyer, an existing group. Reunion organisers face exactly this problem and solve it the same way.
What changes at scale
Wording matters more. A message going to 500 people will be misread by somebody if there is any ambiguity in it, so short and literal beats clever.
Replies matter more too. Even a small reply rate on 500 recipients is a lot of messages coming back, so it is worth deciding in advance who is handling them.
And opt-outs are normal at this size. People leave lists, that is healthy, and every message carries the mechanism automatically. How opt-outs work covers the requirement.