ESR series service routers.ESR-Series. User manual
Specify that the interfaces belong to the security zone:
esr-21-2(config)# interface serial 1/0/2
esr-21-2(config-serial)# security-zone xx 
esr-21-2(config-serial)# exit
esr-21-2(config)#
Objective 2:
Set up IP connectivity between two ESRs on a Serial port, using Dial-Up modems and the Public Switched 
Telephone Network (PSTN).
The ESR-12VF with the following configuration is used as a PSTN emulation:
dialplan pattern factory_test
  description "dialplan for factory test" 
  pattern "S5, L5 (00[1-3]@{local} | [xABCD*#].S)" 
  enable
exit
sip profile 1
  dialplan pattern "factory_test" 
  enable
  proxy primary
    enable
    ip address proxy-server 192.0.2.5
    registration
    ip address registration-server 192.0.2.5
  exit
exit
interface voice-port 1
  sip user phone 001
  profile sip 1
exit
interface voice-port 2
  sip user phone 002
  profile sip 1
  caller-id mode fsk-bell
exit
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Modem compatibility verified
Modem ZyXEL OMNI 56K (MINI)
Modem Acorp-M56SCD