I accidentally took my iphone off airplane while I am in England it is now recognized as an English phone because it pinged off of an English tower. Can I return it to US mode while I am still in England?

I am staying abroad in London for two months. The other day I accidentally took my iPhone off airplane for like, half a second. BUT during that time the phone pinged off of an English phone tower and is now recognized as an English phone. I know this becasue I have been checking my messages through my computer and now I get a busy signal/or message saying basically that the phone is international. I called ATT to verify this. They told me the only way to return it to US mode is when I return to the US, open the phone and it pings off of a US phone tower.

Is this information correct? it seems that there must be a way to return it to US mode while I am still here, no? I am expecting messages from the US to my phone and don't want there to be a busy sigal or error message for the people trying to leave messages.

Thanks!