From outside the UK, replace the trunk prefix 0 of the UK telephone number (mobile or landline) with country code +44.
(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, and many other countries use different prefixes.)
If your mobile supports it (most do), program in any UK numbers (mobile or otherwise) in full international format. Replace the trunk code '0' at the front of the area code with '+44'. (Be sure to include the plus sign!) For example, replace 07700 912345 with +44 7700 912345, or for a landline replace 01632 654321 with +44 1632 654321.
The mobile network, whether in the UK or abroad, will automatically route the call correctly.
If you can't program in the number in international "+44" format, then you should dial the international access prefix of the country you are in (00 in most of Europe, 011 in US/Canada, 0011 in Australia, etc.), and then 44 (for UK), and then the UK number, leaving off the leading '0' trunk code.
There are two ways to call a UK land line from a UK mobile:
The advantage to the first is it's shorter and, for most people, more natural. The advantage to the second is that if you program your contact numbers in international format, they are guaranteed to work when you roam outside the UK.
(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.)
It makes no difference if it is roaming. Dial as normal.
Remove the 0 (zero) from the area code and add +44 e.g. 0204 403766 becomes +44204 403766
Replace the trunk prefix 0 of the UK number with 00 44.
Just dial the UK mobile number. It does not matter that the mobile is roaming in Ireland (or anywhere else); the network will automatically locate the mobile, and the roaming mobile user will pay any applicable roaming surcharges.
Dial the UK mobile number exactly the same way you do when the mobile is in the UK.
Dial 00 353 and then the landline with its area code and leaving out the first zero.
Customers can call 789 from their Virgin Mobile phone to call Virgin Mobile UK. Other customers can call 0845 650 4500.
The mobile virtual network operator Tesco Mobile uses the TelefÃ_nica UK Limited (O2) network as its carrier in UK and Ireland. This enables them to have a 99% coverage of the population in UK and Ireland.
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.
Dial the UK mobile number exactly the same way you do when the mobile is at home in the UK. The mobile network will automatically locate it.
Northern Ireland is in the UK, and the international dialling code for the UK is +44.
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.
To phone anywhere in the UK from Ireland, replace the trunk prefix 0 at the beginning of the UK number with 00 44, or +44 from a GSM mobile. There is one special code. To call a landline in Northern Ireland from the Republic of Ireland, you can dial 048 instead of 00 44 28, followed by the 8-digit local number.
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.