Just to clarify, you wanted to know if the Doro PhoneEasy 623 is dual band?
If yes, then the answer is yes. Dual-band for a GSM mobile phone usually means the 900 / 1800 MHz frequencies. For added information about the phone, please go to: https://www.doro.com.au/doro-phoneeasy-623-optus.html
Hope it helps. Cheers.