UC520 PSTN analog lines and DID

Unanswered Question
Apr 8th, 2009
User Badges:


I'm installing a UC520 with 6 analog lines from the PSTN for 20 DIDs.

I was advised to use command "connection plar opx ###" in the voice-port to configure the DID. This doesn't seem to be the best way as 6 DIDs will use up all my voice-ports. Is there another way to configure the DIDs that will map the PSTN number to an extension as opposed to mapping a voice-port to an extension?

Sorry... I've searched but unless I'm missing something I can't find any documentation on this.

Right now I have the UC520 in a test environment with only one analog line which limits my testing.

Thank you for any help.


  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
vpersaud001 Wed, 04/08/2009 - 09:44
User Badges:

Thank you Marcos. There 8 FXO ports on two cards and a 4-port FXS card - not sure if that's the same as an FXS-DID expansion card.

In CCA I don't see a tab for Inbound Dial Plan - there's a Dial Plan tab with Incoming Call Handling. However the DID translations configuration is disabled with message "DID configuration requires at least one non-FXO trunk to be configured. Note that FXO trunks handle PSTN numbers automatically."

Follow up questions:

1. What is a non-FXO trunk?

2. Will the DID Translation configuration be enabled after connecting more than one FXO?

3. Should there be a separate tab for Inbound Dial Plan?



Steven Smith Wed, 04/08/2009 - 10:00
User Badges:
  • Gold, 750 points or more

There is a card called an FXS/DID card that you could put in the expansion port.  DID service sends the called number down to the line.  The UC500 could then interrupt this and route the call accordingly.  DID trunks are inbound only.  You would know if you had DID service.

I don't think there is a way to do what you want using straight FXO lines.  There is no called number information pushed down the line.  You could try sending all the calls to AA and routing from there, but that probably isn't what you want to do.  You could look into some BRI circuits or maybe a fractional PRI.

vpersaud001 Wed, 04/08/2009 - 12:32
User Badges:

Steven.. Thanks.

The biggest problem is this system is for Mexico and I have to get most of the configuration done and tested at my desk in New York using the one analog line available.

I'm working my way through the labs suggested by Marcos.

For clarification.... to enable DID I need the following:

1. PSTN analog lines must be DID capable

2. FXS/DID expansion card for the UC520 - this will enable the DID configuration in CCA (correct??)


Sorry... I have to add another question. I notice there are only 2 or 4 port DID cards. Is there a way to accomodate more than 4 DIDs? Thanks.

Marcos Hernandez Wed, 04/08/2009 - 12:47
User Badges:
  • Blue, 1500 points or more

Analog DID's are not common in Mexico (rather non-existent). You will likely get analog PSTN lines to connect to FXO ports, meaning you will have to deal with the limitations explained earlier.

I do know that in many cases, a fractional E1 (10 channels, for example) may be more affordable than the equivalent number of analog trunks, as a PSTN option. You would need a T1/E1 MFT expansion card for your UC though, and you would have to configure E1R2 (which is only supported outside of CCA, via CLI). We can certainly help you with this.



Marcos Hernandez Wed, 04/08/2009 - 12:50
User Badges:
  • Blue, 1500 points or more

To answer your other question, 4 analog DID trunks means that you can have up to 4 simultaneous calls but the number of inbound destinations that route to those trunks could be much higher.


vpersaud001 Wed, 04/08/2009 - 14:03
User Badges:

This is the system I have - Cisco UC520-24U-8FXO-K9. Per the documentation the ports (using DID) are 4 FXS/DID, 8 PoE and 8 FXO.

Is the 4 FXS/DID already capable of DID or does that mean it is exclusively DID or FXS? How can I tell by checking the system?

I'm waiting for a response from the provider in Mexico to verify the DID capability of the analog lines. Thanks.

vpersaud001 Thu, 04/09/2009 - 07:22
User Badges:

I figured out the answer to the last question - re built in FXS ports. Please ignore. Thanks.

Moderator Thu, 04/09/2009 - 08:01
User Badges:

Great, thanks for posting!

Cisco Moderation Team

vpersaud001 Mon, 04/13/2009 - 09:57
User Badges:

Hello... please forgive my ignorance on the subject. I have two more questions and sincerely appreciate your help.

1. Is an analog trunk inherently DID capable or is there a difference between the analog trunk and DID trunk?

2. If I have six lines coming in and the DID/FXS card can only accommodate up to 4, how best to optimize the other two lines?

Thanks for your help.

bgrunewald Mon, 04/13/2009 - 20:27
User Badges:

The most common type of analog trunk connects to an FXO port. It is no different from the POTS line in your home - you can plug an analog phone into it and make and receive calls (assuming that it is loop start). CO trunks can also be ground start, which helps eliminate call collisions, which will hang the trunk.

A DID trunk is configured differently, and normally requires a special card in a traditional PBX. In current Cisco products the FXS/DID VIC is configurable in IOS to operate as a DID trunk. DID trunks have several unique attributes:

- They are inbound only

- The CPE (customer) side supplies battery voltage

- After the Telco side goes "off hook" (siezes) the line, the CPE side "winks" by reversing battery on the line for about 200 msec. This is the signal to Telco that the CPE is ready to receive the dialed digits

- The Telco sends the last 3 or 4 digits of the phone number as DTMF digits, which the CPE uses to route the call to an extension

You will need FXO trunks to make outgoing calls. The FXO trunks can also receive calls, but they cannot receive any dialed digits. Because of this, incoming FXO calls are typically PLAR (port makes a call to a predetermined number when it receives a call) connected to an attendant phone or auto attendant.

In a UC520, you only have one slot, so you will only be able to have 4 DID trunks. You could then use the FXO trunks to receive calls to the main published number. If you route the main number to an auto attendant, callers that know the number can still self direct their calls.

I agree with a previous poster - if you can use an E1 card and get fractional ISDN service it will be a lot easier and more flexible. In many cases, 8 or more voice channels are cheaper using digital service than with analog ports, at least in the US.

vpersaud001 Mon, 04/13/2009 - 21:02
User Badges:

Thank you very much for the explanation. Being a total VoIP novice I was getting a little overwhelmed with all the information and trying to figure out which to apply.

I found out this afternoon that the carrier is actually piping in SIP trunks over fiber then breaking out to analog for clients in the complex and it seems it may be better for them to just hand off the SIP trunk to us. So after all the running around to clarify the analog/did trunks I now have to figure out how to configure the uc520 for a SIP trunk.

vpersaud001 Thu, 04/23/2009 - 10:18
User Badges:

Thank you for the link and my apologies for the delayed response - I was waiting for the LEC info but since no response as yet have to now assume it's top secret info.

Should I ever receive the name I'll post it.

The implementation has been delayed until sometime in May. I will update you after. Thanks again for all your help.

Marcos Hernandez Wed, 04/08/2009 - 07:40
User Badges:
  • Blue, 1500 points or more

Are this analog PSTN lines DID-capable? For what you say they must be. Do you have the FXS-DID expansion card?

Assuming the above is correct, then no, connection plar is not the way to do it. On a DID capable trunk, the provider will send you extra digits that will allow you to perform your internal call routing as you please. CCA can be used very easily for this. Go under the Telephony tab and click on Inbound Dialplan. The following labs have detailed information:





This Discussion