How do you send a text message from your cell phone to US?

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.