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Dial Tone Services

All businesses who utilize a Premise or Private Cloud PBX need Dial Tone. Businesses who subscribe to Public Cloud or Hosted PBX services do not need Dial Tone service. There is a big marketing rush to Public Cloud and Hosted PBX solutions but the reality is that only a few percent of businesses are actually migrating. Thus, nearly every business still needs Dial Tone brought into their premise PBX.

There are now four (4) forms of Dial Tone. Any good, modern PBX will handle all four (4) easily and simultaneously.  These four (4) options are POTS, Cellular, PRI and SIP. These are listed in age order, not order of popularity.

POTS – Plain Old Telephone Service

Plain old Telephone Service (POTS) has been around since 1876 and is still the most stable form of Dial Tone. POTS is often required by local building codes for alarm and elevator emergency lines. Each POTS has a unique phone number and unique physical address enabling first responders to know exactly where a call is originating. Many companies still deploy them for fax machines. There is a push by many Local Exchange Carriers (LEC) to eliminate these lines because the equipment terminating them in the Central Offices (CO) are very old and failing. It is also very expensive for the LEC to maintain the copper wires on the telephone poles. Be sure to check with local building codes before eliminating these lines for alarms and elevators.


Cellular circuits have been around since 1979 and they utilize the cell phone network. These circuits may be interconnected to an alarm panel, elevator phone or a Premise PBX. They operate just like a POTS circuit in that each one has a unique phone number. They can be interfaced with the system as POTS or SIP circuit and are becoming very popular for use in alarm panels and elevator phone applications. There are some customers who use them for a backup to their primary Dial Tone option. They can be purchased from nearly every cell phone carrier and there are multiple options for connecting them to Premise PBX systems.

PRI – Primary Rate Interface

The Integrated Services Digital Network (ISDN) Primary Rate Interface (PRI) circuits have been around since 1988 and is still a very common form of Dial Tone delivered to businesses. The PRI utilizes an ISDN circuit to provide 23 talk paths and 1 data path. There are two big benefits to using at PRI verses POTS. First, the PRI is about ½ the cost of 23 POTS lines. The second, even bigger benefit is the ability to deliver an unlimited number of Direct Inward Dial (DID) numbers on the PRI. This technology enables businesses to deliver unique phone numbers (DID’s) to each person, department or location in their business.

The challenge with using this technology for alarm lines and elevator lines is that a Premise PBX must sit between the phone and the first responders. If the PBX has a problem, the alarm may not go through. Additionally, since PRI’s can be centralized, a remote site user may be using it for outbound dialing. If the remote site user dials 911, the first responder may see the address of the PRI, not the address of the person dialing 911. There are many solutions to this problem, be sure to discuss with your Premise PBX support team.

SIP- Session Initiation Protocol

Session Initiation Protocol (SIP) has been around since 1996 and was standardized (sort of) as Request For Comments (RFC)-2543 in 1999. It was in November 2000 that SIP was accepted as a 3GPP signaling protocol and a permanent member of the IP Multimedia Subsystem (IMS) architecture for IP-based streaming multimedia services in Cellular networks. Finally, in June 2002 the specification was revised in RFC-3261 and various extensions and clarifications have been published since.

SIP is the preferred delivery method by nearly all Local Exchange Carriers (LEC). They bring the Dial Tone in on a data circuit, fiber or coax, and deliver it to the Premise PBX in whatever format the PBX can handle. These delivery formats include POTS, PRI and SIP direct. Delivery as POTS and PRI work identical to POTS and PRI described above. Delivery as SIP requires a Session Border Controller (SBC) to sit between the dial tone and the Premise PBX. This SBC is sometimes deployed by the SIP provider but most often is a required purchase by the customer. The SBC is a SIP Firewall and much more, and it is a very important piece of equipment. SIP Dial Tone also has a risk for dialing emergency services for 911, alarm lines and elevator phones. Careful considerations are required with purchasing SIP Dial Tone. SIP Dial Tone delivery is where all LEC’s are going because it is simple to deploy and manage.

Long Distance

Long Distance services include IntraLATA, IntraState, InterState and International dialing. Essentially, the customer has to choose a Long Distance (LD) Carrier. Many customers default to their Local Exchange Carrier (LEC) as their LD Carrier. Because the “world” is not wired by each and every LD Carrier, the LD Carrier of the dialing party must “pay” all other LD Carriers between the call origination and call destination. Thus, they have to collect a fee from the customer.  This fee is usually a very small fee, billed in 6 second increments. Many LEC’s bundle a large amount of free LD in their fees.