In general it's always best to store numbers in the "+country code" format. EG the UK Virgin Mobile customer services call centre should be stored as +44-345-6000789. Where 44 is for the UK. Your friend in France is +33-n-nnnnnnnn and so on.
The "+" is an instruction to the handset to prefix the call with the international access code for whatever country you are actually calling from. When placing a call from Europe the "+" translates to 00. However 00 is far from universal. EG for countries using the North American numbering plan "+" needs to translate to 011, that applies to the USA, Canada and most of the Caribbean. Japan uses 010 and so on.
So in my example above if you were in Jamaica and wanted to call VM customer services in the UK, the handset would automatically translate the number stored in your phone-book to 011-44-345-6000789. Likewise the number for your French friend would correctly translate to 011-33-n-nnnnnnnn.
If you had simply stored the VM number as 0345-6000789 and tried to call it from anywhere other than the UK the call would fail as there is nothing to indicate that the call is international and nothing to indicate the required country.
If you had stored the VM number as 00-44-345-6000789 the call would succeed from France as the international access code (ie 00) is correct for France. However it would fail from the USA as the international access code is incorrect.
If your contact in France has a mobile then consider using Whatsapp, especially if you are both connected to Wi-Fi. That will give you free calls. I use it successfully to the USA and Europe.
Just checked my VM bill on line and unfortunately I have been charged 90 p for a 38 second call to a French landline from my VM mobile in the UK. So beware VM misleading information when you ring 789 from your mobile and check your costs after 15/30 mins.
The 90p is the minimum charge. That's because "Our calls are billed per second and our charges are rounded to the nearest penny. A minimum call charge of one minute applies if you pay by Direct Debit or have a Pay Monthly contract".
The irony is that because of EU roaming caps it would only cost you 4p/min to ring the UK from France on your mobile. I would look at using Whatsapp over Wi-Fi or mobile data. I call the USA and several European based mobiles on a regular basis. Whilst not guaranteed, calls are usually fine.