You need to activate the roaming service first; you can do that (ideally before you leave the UK) by calling the international customer care number of your mobile provider which you will find on their website.
To dial the call, just replace the leading 0 of the UK domestic number with the UK country code +44 (including the plus symbol). For example, 01632 960123 becomes +44 1632 960123.
(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)
The International code for the UK is +44, replace the first zero of the mobile number you want to call with +44. (eg Mobile no 0780 117 0000 becomes +44780 117 0000)
By dialling the number
You can't.
Customers can call 789 from their Virgin Mobile phone to call Virgin Mobile UK. Other customers can call 0845 650 4500.
If the US mobile phone is in international roaming mode, you may be able to dial its normal US number. If the phone has swapped in a UK SIM card, you will need to dial the UK number.
To call a UK-based mobile phone, wherever in the world it may be physically located, you always just dial the UK mobile number the same way you do when the mobile is in the UK. The network will automatically locate the mobile and re-route the call accordingly, and the mobile user will pay any applicable surcharges for roaming.
To call a UK mobile from a UK landline, you always just dial the UK mobile number, exactly the same way you do when it is in the UK. The network will find the mobile automatically, and the mobile user will pay any applicable surcharges for international roaming.
The fact that the UK mobile is in France has no bearing on the cost to the original caller. The caller will pay the normal charge for a call to a UK mobile number, and the mobile user will pay any applicable surcharge for the international roaming.
The mobile phone number remains the same no matter where you take your phone - callers do not have to guess where your phone is before they dial you. In this case, the US caller would call you in the same was as calling any other UK telephone number: international access code 011, then country code 44 for the UK, followed by the full mobile number ignoring the leading zero. For example, 07345 345678 would be dialled as 011 44 7345 345678 from the USA.
Dial: 011 = international access prefix from the US 44 = country code for the UK 7XXX = mobile code (7400 through 7999) (6-digit mobile number) # = pound sign tells the telephone switch not to wait for more possible digits
To call a UK-based mobile phone, anywhere in the world that it may be physically located, just dial the UK mobile numberexactly the same way you do when the mobile is in the UK.
Assuming it's a contact in the phone's memory - just do exactly as you would if both phones were home in he UK. The network will sort out the logistics and connect both phones.
Yes, it is an international call. The rates depend on the calling plan you have with your mobile service provider.
In general, a UK mobile user does not pay for incoming calls, no matter where they originate.