How do you call a UK number when both phones are in the us?

To call a mobile phone that is roaming outside its home country, you dial the number the same way you do when the mobile is at home.

To call a UK mobile phone from any GSM mobile, dial +44 followed by the UK number, omitting the trunk prefix 0. For example, 07700 900123 becomes +44 7700 900123.

If you are calling from a US landline, or from a non-GSM mobile, replace the plus symbol with the international access prefix (011 from the United States).

(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.)