Before you start

An unlocked handset, or an eSIM-capable one
A US address for postpaid, but not for prepaid
A payment method — prepaid accepts foreign cards, postpaid usually does not
A plan whose Canada roaming terms you have actually read, if you live near the river

Step-by-step

1
Activate an eSIM before flying, or buy prepaid on arrival
A US eSIM activates before you land so you have a working number immediately for rides, deliveries and bank verification codes. No identity registration is required at any point.
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2
Check your handset is unlocked and band-compatible
Phones bought on contract abroad are frequently locked, and US band combinations trip up some regional models. With 3G shut down there is no fallback network to land on.
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3
Start prepaid rather than fighting a credit check
Postpaid runs a credit check. With no US file you will be declined or asked for a deposit. Prepaid has no check and is usually cheaper for the same data allowance.
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4
Check the Canada roaming terms before you go near the river
Several US plans include Canada and Mexico at no extra cost; several do not, and prepaid plans frequently do not. Along the Detroit riverfront, on Belle Isle and on Jefferson Avenue, handsets routinely register on Windsor towers. If your plan excludes Canada, turn off international data roaming or expect charges for a walk you took entirely in America.
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5
Lock the network manually if it keeps happening
If your handset repeatedly prefers a Canadian tower at home or at work, set the network selection to manual and pick the US carrier, rather than leaving it automatic. It is a settings change, not a plan change.
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6
Port rather than cancel if you switch later
Number portability is a legal right and works between carriers and MVNOs. Request the port from the new carrier — never cancel the old service first, or you lose the number.
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Documents you’ll need

Passport for postpaid; nothing at all for prepaid
US address for postpaid
A payment card — prepaid usually accepts foreign cards, postpaid rarely does

Things most newcomers don’t know

You can roam into Canada without leaving Detroit.

The border is the river, and Windsor's towers reach across it. People on the Riverwalk, on Belle Isle and along Jefferson Avenue find their handsets registered on a Canadian network and get billed for roaming on a walk that never left the United States. If your plan excludes Canada, disable international data roaming or lock the network manually.

Source: community-reported

Plans that include Canada are worth more here than the price difference.

In a metro where crossing for dinner, a concert or a cheaper flight from Toronto is normal, a plan bundling Canada removes a recurring irritation rather than covering a rare holiday. Compare on that inclusion, not only on the data allowance.

Source: community-reported

The credit check is the barrier, not the documentation.

Carriers check credit before offering postpaid, and a newcomer with no file gets declined or charged a deposit. Prepaid sidesteps it entirely and usually costs less for the same data.

Source: community-reported

The US has no SIM registration at all.

Most countries in this app require passport registration to activate a SIM. The US requires nothing whatsoever for prepaid — you can buy one in a supermarket and be online in minutes.

Source: FCC — consumer guides