PSTN Call Switching

The preceding sections explored the various PSTN trunk types, signaling methods, and router hardware you can use to connect to the PSTN. But there are more considerations than just physical connectivity. One thing to keep in mind is that the PSTN numbers and your internal extension numbers are almost certainly not the same, at least not the same length. Digit translation must occur to map one set of numbers to another. You'll learn more about this in the later section "Digit Manipulation."

Another consideration is what PSTN numbers (and how many) your business has or needs. Do you have just one main office number, and the receptionist directs all calls to the correct employee? Do you need an AA menu to have callers switch to the person or service they want to speak to? Should you have DID numbers for all or some of your employees? Do you prefer a Key System type of operation, where a series of PSTN numbers appear as distinct line appearances on a number of phones, and any employee can pick up any call? There is interaction between your business needs, the PSTN service you get from your provider, the capabilities of the physical connection to the PSTN, and the Cisco CME configuration (IP phone button appearances) to use.

The physical connection is likely dictated in large part by cost, your office's geographic location, and the number of voice channels your office needs. You may desire DID service for your business. However, if it's not offered in your area at a cost-effective level, you have little choice but to settle for non-DID service.

The following sections explore considerations about how calls may be routed depending on whether you have DID service and, even if you do, how you can handle calls to non-DID destinations within your business.

PSTN Call Switching with DID Enabled

Many offices deploying Cisco CME have DID capability from the PSTN provider for some subset of employees. PSTN calls to DID destinations can be switched automatically to the employee's phone without any manual intervention.

There are two situations to consider, depending on how DID numbers are allocated to destinations within your business:

• All employees, the AA, and voice mail pilot numbers have DID numbers assigned. In this configuration, PSTN calls can be switched as follows:

  The main office number (non-DID calls) terminates on the AA pilot (for AA assistance) or on an IP phone extension (for receptionist assistance).

  Employee DID numbers terminate on the extension for that person's IP phone.

  The number employees call from PSTN locations to check their voice mail terminates on the voice mail pilot number.

• Some employees have DID numbers assigned, and others do not. In this configuration, PSTN calls are handled as follows:

  Calls to the main office number and for PSTN voice mail checking are handled as per the preceding scenario.

  Calls to employees with DID numbers terminate on the extension for that person's phone.

  Calls to employees without DID numbers terminate on the AA (or receptionist's extension). These callers then can dial through or be transferred to the extension of the person they want to reach.

PSTN Call Switching with DNIS But No DID

PSTN call switching with DNIS and no DID is not a likely configuration, but it is possible. In this configuration, your business does not have DID service and, therefore, has only a single main office number from the PSTN provider even though you have multiple trunks. Or even if you have multiple PSTN numbers, they are not associated with particular employees, but instead are just alternate main office numbers.

Although the dialed number is delivered via DNIS from the PSTN to the PSTN gateway, it is of little use to switch calls to individual destinations. In this situation, you have two configuration choices:

• Regardless of the dialed number, all PSTN calls are terminated on the AA pilot (for AA assistance) or on a specific phone extension (for receptionist assistance).
• All PSTN lines appear on multiple phones (Key System operation). Any employee at these phones can answer any of the lines, regardless of what number the caller dialed.

If all calls are directed to the AA (or a receptionist), caller-busy conditions must be carefully considered. For example, you need to determine what should happen if all AA ports are busy or all the receptionists are busy. If you do not want busy tone returned, more ports or receptionists may be required, or alternate destinations to switch calls to (lower-preference dial peers) or DID service may be needed for high-volume destinations in your business.

PSTN Call Switching with No DNIS (FXO Trunks)

If the office has only FXO trunks, no DNIS (or DID) capability is technically possible. This scenario is very common for a small standalone office or a small branch of a bigger network that has only a few business lines from the local CO.

Because no dialed digits (DNIS) are available on FXO trunks, these calls must be autoterminated on a predetermined destination, most often the AA or the receptionist's extension. This can be achieved with a private line automatic ringdown (PLAR) configuration on the voice port where a particular destination extension is associated with the trunk, and all calls arriving on that trunk are switched as if they had dialed the configured extension. This syntax is shown in Example 6-8, where all calls arriving on the FXO trunk on slot 1/0/0 are switched as if they had dialed extension 6800.

Example 6-8. PLAR Syntax for FXO Trunks

Router#show running-config
voice-port 1/0/0
 connection plar opx 6800 

Most small offices have multiple FXO trunks to the PSTN because each trunk can carry only a single call. One or more PSTN numbers may be associated with these trunks or this trunk group, depending on the PSTN service the business subscribes to. Generally, there are two possibilities:

• A single PSTN main office numberIn this configuration, all calls from the PSTN are terminated (via the PLAR feature) to the AA pilot (for AA assistance) or to a specific phone extension (for receptionist assistance).
• Multiple PSTN numbersYou could have one PSTN number for the main office and another for voice mail retrieval by employees. In this case, calls on main office trunks are switched as in the preceding case, and calls to voice mail are switched (via PLAR) to the voice mail pilot number. Clearly, these two types of calls must be delivered on different physical trunks or trunk groups so that each is autoterminated to the correct destination.

Sharing all FXO trunks across all PSTN calls (the first case in the preceding list) results in better trunk utilization than assigning distinct FXO trunk(s) to the main office number and other distinct FXO trunk(s) to the voice mail pilot number (the second case).

Digit Manipulation