How do you call Bangkok mobile number from UK mobile?

Thailand uses the country code +66, and you must also drop the trunk prefix 0 from the beginning of the Thai domestic telephone number. Using a mobile phone, just dial the number in international format, beginning with the plus symbol.

If your mobile does not allow you to enter the plus symbol in a telephone number, or if you are calling from a landline, substitute the UK's international access prefix 00 for the plus symbol.