To dial a UK mobile from the UK, just dial the mobile number as you normally would, whether the mobile user is at home or roaming internationally.
To dial a UK mobile number from a UK mobile that is roaming internationally, the most reliable method is to dial the number in full international format: replace the trunk prefix 0 with country code +44 (including the plus sign).
Alternatively, in some areas you may (or may not) be able to dial the UK mobile number in domestic format; however, doing so may also connect you to a number in the country where you're roaming, if the number ranges overlap.
A UK-based mobile phone has a telephone number beginning with +44 74 through +44 79. In domestic format, that's 074xx through 079xx.
From any GSM mobile, anywhere in the world, dial the full number in international format, beginning with the plus symbol. For example, (fictitious) UK mobile number 07700 900123 would be dialed as +44 7700 900123.
From a landline in Spain, dial the international access prefix 00, followed by the international number; for example, 00 44 7700 900123.
If a mobile phone from some other country is physically in the UK, you dial its number the same way you do when the mobile is in its home country. The network will automatically locate the mobile, and the mobile user will pay any applicable roaming surcharges.
The country code for the United Kingdom is +44. Within the UK, you just substitute the trunk prefix 0 for the country code +44, so the number above is 0808 xxx xxxx, which is toll-free from a landline.
(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.)
To call a Spanish mobile phone, dial 00 34 and then the Spanish number.
To call a UK mobile phone that is currently in Spain, dial the UK mobile number the same way you would when it is in the UK.
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.
Just as you would call an English landline.
Where the UK mobile number is 07xxx xxxxxx dial either +44 7xxx xxxxxx or 00 44 7xxx xxxxxx. It doesn't matter which country the other phone is being used in.
+34 (including the plus symbol) followed by the 9-digit Spanish number
Dial the UK mobile number in full international format: replace the trunk prefix 0 with telephone country code +44 (including the plus symbol).
England is the country name. People from England are English. Things from England are English. The language of England is English.
+44 is the international code for England.
The physical location of a mobile phone is not relevant to how you dial it. To call a US-based mobile phone from the UK, dial 00 1 and the area code and number.
Vodaphone
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.
To call Spain from any GSM mobile, anywhere in the world, dial +34 and then the 9-digit Spanish number (landline or mobile).
English