Any call placed from your phone (with cingular's sim in it) while there will be charged whatever the roaming rate is. The phones in your hotel room will probably not be free either. It might even be more expensive than Cingular's roaming rate. If there are pay phones there, thet will probably be your best option. Some calling cards also levy a surcharge for using a pay phone. Also, as posted my the experts here, a UK calling card will be a whole lot cheaper than the Sams/Costco calling card you take from here.
Here is the sams card. You probably will need to contact the CSR to get the rates from there to US, and the number to dial from there.
http://www.samsclub.com/shopping/navigate....3786&pCatg=5875
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile