Hello Alsayed,
we can say that E1/T1 PRI is used when you connect to ISDN world via a primary rate interface.
E1/T1 CAS channel associated signaling can be used in voice environments to connect to digital trunks that are not ISDN
the multiflex is a type of interface that is flexible and can be configured as E1 or T1
To be noted that to connect to ISDN or to use for VoIP you may need DSP resources on the routers or you get an error message telling that you lack DSP resources
Hope to help
Giuseppe