Caller ID test on your HT503:
At first, we have to be sure that the HT503 handles the caller ID correctly; otherwise there will be no need to proceed to the next step. A verification to see if the HT503 handles the caller ID properly is needed.  You can verify if the caller ID is properly handled by doing like this:
Open the web interface of your HT503.
Click on the FXO port
Scroll down of the page:
Go to the Caller ID Scheme: Select the Caller ID Scheme that is supported by your phone service provider from the drop down menu
Go to Caller ID Transport Type: Select the method by which the caller ID is transported.
Scroll down of the page to Number of rings. This is the number of times the FXO port will be ringing before having the a VoIP extension ringing or before having the phone on the FXS port ringing if PSTN Ring through FXS is set to yes. Please set it to a value that is equal or greater than 4.
Scroll down of the page to PSTN Ring Thru FXS to Yes, so the phone connected to the FXS port will ring.
Click on update
Check if the same Caller ID settings on the FXS port are similar to those on the FXO port.
Go to the Basic Settings , and check if the Life Line Mode is set to Auto. Setting the Life Line Mode to Auto will allow the device to parse the Caller ID if the HT503 is on, and automatically be a relay if the device is off.
Plug the phone to the FXS port, and do a PSTN call to the HT503.
If you see the caller ID on your phone and the HT503 is on and all the previous parameters are respected, the HT503 handles properly the Caller ID. And now you  can move on to the next step.
Configuration of your HT503:
Now that you are sure that the HT503 is handling the Caller ID correctly, you should be able to do the peering of the HT503 with Elastix and having your CDR fetched with the Caller ID from the PSTN network and from VoIP to PSTN. As mentioned previously, the peering is without registration.
Please follow the steps below so you can configure your HT503 to route a call to an extension that you want to receive the call on. The settings below are either an addition or a change to the configuration that was made previously, so you can go through the Asterisk server to make the call to the FXS port of the HT503. Please make sure you go through all these settings:
Page
Settings
Basic Settings
Unconditional Call Forward: this option will allow you to forward a PSTN call to any extension that you want in the VoIP network. You can configure the HT503 to send the call to an extension that is registered in Elastix. Example: your Elastix PBX IP address is 192.168.1.254 and listening on incoming SIP connections on 5060, the FXS port of the HT503 is registered to Elastix and the extension number is 4001. So the Unconditional Call Forward will be 4001@192.168.1.254:5060
Advanced Settings
Life Line Mode: as explained before this will change the behavior of the HT503 depending on whether it’s powered on or off. If the device is on, then It will handle the Caller ID and can transfer it to the Asterisk server, if not it will act just as a simple relay. But this time, it should not ring through the FXS port as the HT503 is set not to ring through it.
FXO Port
Account Active set to YES
Primary SIP server set to the IP address of the PBX
User ID is phone number set to NO
SIP Registration set to NO
Unregister on Reboot set to YES (this will allow the HT503 to clear all the SIP credentials that were set to be used on the device before and unregister from the server)
Outgoing Calls without Registration set to YES (this will allow to make PSTN call without being registered to a SIP server)
Number of Rings is greater than or equal 4
Stage Method set to 1
PSTN Ring through FXS set to NO
FXS Port
Verify if the caller ID settings are the same as the ones defined on the FXO Port.
Register the FXS port on the Asterisk /Elastix server so you can use it with the phone
Configuration your Asterisk PBX (Elastix):
Please follow the steps below to configure your Asterisk IP PBX:
1 -Add A SIP Trunk from the Trunk men
2 -Under the outgoing settings:
A – Set a trunk name (i.e. HT503_Trunk)
B – For the PEER DETAILS enter the following:
host=IP_ADDRESS_OF_YOUR_HT503
type=peer
canreinvite=no
insecure=very
dtmfmode=rfc2833
nat=yes
port=5062 (this is the port number used by the HT503’s FXO port)
3 -Under the incoming settings:
A- Give the User Context to a certain name.
B-For the USER DETAILS enter the following:
context=from-trunk
host=dynamic
insecure=very
type=friend
dtmfmode=rfc2833
4 -Submit and Apply the settings.
5 -Go to the Outbound Routes:
A – Give the outbound route a name
B – Set the Dial Pattern if you want to dial a PSTN number.
C – Under Trunk Sequence, select the HT503 trunk that you have created before.
6 – Submit and apply the settings.
Final Result:
Now you should be able make and receive calls from and to a PSTN network, while having all the calls logged into the CDR of your server.