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 station, 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
Pick a plan on its calls-to-the-EU line, not just its data allowance
EU rules mean your French allowance works while you are in Germany. They say nothing about phoning a German number from your flat in Strasbourg, which is an international call unless the plan includes calls to the EU. If your work, your children's activities or your weekend life run partly in German, this line item matters more than the data.
OnlineWho: You
3
Fix the network manually if your phone keeps registering on a German mast
Radio does not respect the Rhine, and handsets in Neudorf, the Port du Rhin and the eastern districts routinely register on German networks indoors. Within EU allowances this normally costs nothing, but it confuses short-code numbers, some two-factor systems and anything that infers your country from the network.
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, and noticeably cheaper than the German equivalent. 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 Eurométropole is good and prices are low. Installation takes one to three weeks and may need a technician and access to the riser. In the Neustadt's monumental apartment blocks and the half-timbered stock of the Grande Île, what is already installed in the building matters more than the street map.
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 Germany and calling Germany 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 France to a German number. Someone living in Strasbourg and phoning a Kehl doctor, a German school or a colleague in Offenburg is making international calls from home. Several French plans include calls to EU landlines and mobiles for a euro or two more, and over a year the difference is real.

Source: Arcep / EU roaming rules

Your phone will cross the Rhine without telling you.

German masts cover much of eastern Strasbourg, and handsets register on them indoors without any prompt. Within EU allowances that is usually harmless, but it breaks French short codes, confuses some two-factor systems and can make services think you are abroad. Setting network selection manually to a French operator takes thirty seconds and removes a class of intermittent problems that are otherwise very hard to diagnose.

Source: community-reported

French mobile is much cheaper than German mobile, which matters if you priced the move from the German side.

The 2012 price war permanently reset French tariffs, and France now sits among the cheapest markets in western Europe while Germany remains among the more expensive. If you are moving from Germany or comparing a Karlsruhe offer against a Strasbourg one, the mobile and fibre line in your budget should come down rather than stay flat.

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 German numbers.
Letting the handset sit on a German 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 Grande Île or Neustadt 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.