Yes, provided you have international calling enabled. Just enter the number with country code +1 (including the plus symbol) and the 3-digit area code and 7-digit number.
(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.)
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.
The same way you do with any other phone number.
From the UK, dial the International Access Code 00, followed by the US Country Code 1, followed by the normal US area code and US local number.
Just like you would to any other place - by using the right telephone number. For instance if you're texting from the UK to a US phone, you need to use the US country code, and to drop the first zero in the cell phone prefix. If you're texting to a UK phone in the US, simply use the same number as if the phone was in the UK. The network jeeps track of where the phone is. Note that some contracts and prepaid phones might not allow international traffic.
To block your Caller ID, you dial the prefix *67 (or 1167) if you're in the US or Canada, and 141 if you're in the UK or Ireland. Other countries use other prefixes.
you have to have long distant calls and also know a phone number that leads there
If you're in the US: 18005220020 For UK it's 08444 992 992
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.
I think that the phone number is still the same so you would call it as if it is in the US. To answer my own question, (they have been here and now gone home again), you dial +1 followed by cell phone no.
Contact your mobile phone company and ask them for advice. They may be able to block a specific number, or give you a new number.
The most reliable method is to enter the US number in full international format, which is to say, just prefix it with +1 (including the plus symbol).
First, you need the UK telephone number for Scotland Yard. Then just replace the leading 0 of the UK number with +44 (from a mobile phone) or 011-44 (from a landline).