There is no specific requirement for any particular transmission mode on 900,1.9,2.4, and 5.8, but in practice, virtually all newer 900 MHz phones are inexpensive analog models with digital features such as DSSS and FHSS generally available only on the higher frequencies.
There is no specific requirement for any particular transmission mode on 900.900 is 1.9.900 is 2.4.900 is 5.8. Virtually all newer 900 MHz phones are inexpensive analog models with digital features such as DSSS and FHSS generally available only on the higher frequencies. This is in practice.