i have 2921 gateway with PVDM3-64 and 12 FXO ports. I am using the gateway as SRST and for PSTN calls. I have a confusion about the DSP calculation for e-phones (are they utilizing any DSP resorce and how much?) note that i have max-ephone 25 and max-dn 50
are the DSP resources utilized all the time in case of SRST active and in case the phones are registered to the call manager cluster?
Ephones have no bearing DSP usage. DSP's are utilized when an IP leg needs to be translated to a POTS leg, or for the use of hardware conferencing/transcoding. So, the number of FXO lines will factor into the number of DSP's needed, as will the need for any hw conferencing.
You can check your DSP needs with the following tool: