Lets enable the ISR to accept SIP registration requests and act a SIP Proxy

voice service voip
  sip
   registrar server

Also we will need to allow the ISR to connect SIP to SIP endpoints or call legs.

voice service voip
  allow-connections sip to sip

Lets look at enabling SRST for SIP Endpoints. Enter into the SIP Telephony Mode

voice register global

Default SIP Telephony mode is SRST mode, so we need do not need to change anything here, however the command is below.

no mode cme

We now configure the license restrictions for DNs and Devices (Pools) . Below I have a license limit of 25 pools and 100 DNs.

max-pool 25 (You will need to accept the UAL)
max-dn 100

Optional we can also configured a System Message, this message displays above the Softkeys when in SRST Mode.

system message Phone is Fallback Mode

Optional I also like to keep interdigit timeout to a minimum, I like to configured this to 7 seconds.

voice register global
timeouts interdigit 7

Configure a Voice Register Pool for Endpoint Configuration to merge into.

First configure a codec voice class if not already configured.

voice class codec 1
  codec preference 1 g729r8
  codec preference 2 g711alaw
  codec preference 3 g711ulaw
Now configure the Voice Register Pool

voice register pool 1
  id nework 172.20.100.0 mask 255.255.255.0
  voice-class codec 1
  dtmf-relay rtp-nte sip-kpml

Save the ISR configuration and should be ready to fallback SIP endpoints into SRST mode.

Summary Configuration

voice register global
timeouts interdigit 7
system message Fallback Mode is Active
max-dn 100
max-pool 25

voice register pool  1
id network 172.20.100.0 mask 255.255.255.0
dtmf-relay rtp-nte sip-kpml
voice-class codec 1

*Note: To register 8900 series endpoints, you must be running CME/SRST version 9.1