Before you start

Passport, for a prepaid SIM
A French RIB, for most contract tariffs
An unlocked or eSIM-capable handset
A French address, for fibre installation

Step-by-step

1
Buy a prepaid SIM on your passport the day you arrive
Available at the airport, in phone shops and in tabacs. Identity verification is required and a passport satisfies it. A French number in your first hours unlocks bank verification codes and appointment systems.
In personWho: YouDay 1
2
Check what the plan includes for calls to Belgium, not just for roaming in Belgium
EU rules mean your French allowance works while you are physically in Belgium. They say nothing about calling a Belgian number from your flat in Lille, which is an international call and priced as one unless your plan includes calls to the EU. If you will be phoning Belgian colleagues, schools or landlords daily, pick the plan on that line item.
OnlineWho: You
3
Turn off automatic network selection if your phone keeps jumping the border
Masts do not respect frontiers, and phones in the northern communes routinely register on Belgian networks while sitting at a French kitchen table. Within EU allowances this normally costs nothing, but it can confuse two-factor codes, local-rate numbers and anything geolocated. Setting the network manually to a French operator fixes it.
Mobile appWho: You
4
Move to a sans engagement contract once you have a RIB
French no-commitment tariffs are month-to-month and remarkably cheap for large data allowances. There is little reason to take a long commitment unless a subsidised handset genuinely justifies it.
OnlineWho: You
5
Order fibre as soon as you have an address
Coverage across the métropole is good and prices are low. Installation takes one to three weeks and may need a technician and access to the building's riser. In the old brick stock of Vieux Lille, what is already installed in the building matters more than what the street map shows.
OnlineWho: You
6
Keep your number when you switch operators
Request your RIO code by calling 3179 free of charge from the line you want to port, then hand it to the new operator. Never cancel first — you lose the number permanently.
Mobile appWho: You

Documents you’ll need

Passport, for prepaid registration
French RIB, for a contract
Justificatif de domicile, for fibre installation

Things most newcomers don’t know

Roaming in Belgium and calling Belgium are two different things, and only one of them is free.

The EU roam-like-at-home rules cover you when you are physically in another member state using your home allowance. They do not cover an international call placed from your own country to a foreign number. Someone living in Lille and phoning a Belgian office, school or landlord every day is making international calls from home, at whatever their plan charges. Several French tariffs include calls to EU landlines and mobiles for a euro or two more, and the difference over a year is significant.

Source: Arcep / EU roaming rules

Your phone will cross the border without telling you.

Radio does not stop at a frontier, and in the northern communes of the métropole a handset will happily register on a Belgian network while you sit indoors in France. Within EU allowances that is usually harmless, but it interferes with short-code numbers, some two-factor systems and services that infer your country from the network. Fixing the network selection manually takes thirty seconds and removes a category of intermittent, hard-to-diagnose annoyance.

Source: community-reported

French mobile is cheap in a way that surprises almost every arrival.

The entry of a disruptive fourth operator in 2012 triggered a price war whose effects never reversed, and France now has some of the lowest mobile tariffs in western Europe — noticeably cheaper than Belgium, which matters if you were pricing your move off a Brussels quote. Large-data plans with no commitment cost a fraction of the UK or German equivalent.

Source: Arcep

The RIO code is how you keep your number, and you request it from the line itself.

Call 3179 free of charge from the phone you want to port and a voice server reads you a RIO code and sends it to you by SMS. Hand that to the new operator. They run the switch and close the old contract automatically. Cancelling the old service first loses the number for good, and very few newcomers know the number exists — it is the most common self-inflicted telecom mistake in France.

Source: Arcep

Common mistakes to avoid

Assuming EU roaming rules cover calls made from France to Belgian numbers.
Letting the handset sit on a Belgian network at home and blaming the operator for odd behaviour.
Signing a two-year commitment when no-commitment tariffs are cheap and switchable.
Cancelling your old service before requesting the RIO code and losing the number.
Assuming fibre reaches a Vieux Lille flat because the street is covered.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.