KB112 - Voice Configuration & Signaling Options


Clearfly offers several different phone service interconnection options for our customer and integration partners.

Legacy Connections

Although at Clearfly we pride ourselves on our SIP expertise and experience, native SIP trunks and subscribers are only one tool in our interconnection toolbox. In fact, a significant percentage of our customers are actually served via legacy analog (FXS), ISDN PRI or CAS/RBS T1 gateways. These managed gateways connect back to Clearfly via SIP, but from the end-user’s perspective the circuits we deliver are indistinguishable from the legacy service that is being replaced.

FXS (Analog)

An FXS hand-off simply means that Clearfly is delivering a standard copper phone line for a customer’s legacy phone system, key system, modem, credit card machine or even just a standalone phone. Analog service from Clearfly requires that the customer install a managed voice gateway at their premises. This device can be connected directly to the Internet via a built-in Ethernet connection or can terminate managed Clearfly DSL, T1 or Ethernet circuits. Clearfly’s demarcation point for analog service is typically a 50-pin female amphenol connector or a 66-block pre-wired with a 50-pin male amphenol stub. All Clearfly FXS interfaces are loop-start by default, but can be configured as ground-start at the customer’s request.


CAS/RBS T1s are T1 digital circuits that are broken up (channelized) into 24 individual 64kbps voice paths (channels). The robbed-bit portion of RBS refers to a mechanism by which one out of every six frames has seven instead of eight bits, with the missing payload bit instead used (robbed) for signaling on the trunk. As with Clearfly’s analog service, RBS hand-off requires installation of a Clearfly-managed gateway. Technical details for Clearfly’s RBS trunks are as follows:

  • Linecoding: B8ZS

  • Framing: ESF

  • Signaling: E&M

  • Supervision: immediate start, wink start or FGD

    • Wink start is configured by default

    • FGD is required for caller ID delivery via DTMF (inband MF signaling is not supported)

  • Dialtone: Enabled by default, but can be disabled

  • Inbound Channel Selection: Ascending


PRIs are T1s that utilize ISDN signaling to deliver 23 voice (bearer) channels to a phone system. Unlike an RBS T1 in which the signaling is in-band in the voice channels, PRIs utilize a dedicated 64kbps digital channel to signal for the 23 voice channels. As with Clearfly’s analog services, PRI hand-off requires installation of a Clearfly-managed gateway. Technical details for Clearfly PRIs are as follows:

  • Linecoding: B8ZS

  • Framing: ESF

  • Supported ISDN Switch Types: National ISDN II, ATT 4ESS, Lucent 5ESS, Nortel DMS-100

    • National ISDN II (NI2) is the default

  • D-Channel: 24 (cannot be changed)

  • B-Channels: Default is 23 (configurable)

  • Inbound Channel Selection: Ascending

  • Name Delivery (Caller ID): Setup message, Facility IE (configurable)

SIP Trunks

Although the underlying technology is vastly different, SIP trunks are functionally very similar to PRIs in that they have a dedicated out-of-band signaling path that controls the associated voice (media) channels. However, whereas PRIs are limited to 23 concurrent calls by the channels available on a T1, SIP trunks are limited only by the amount of bandwidth available to a phone system. With SIP a single signaling path can now control the setup, teardown and maintenance needs of hundreds or even thousands of simultaneous calls.

Static SIP Trunks

With a static trunk, Clearfly manually configures our switch and SBC to allow SIP signaling traffic to/from a customer’s unique static public IP address and UDP port. This unique IP/port combination is provided to Clearfly by the customer during the pre-provisioning process. If the customer’s IP address or SIP signaling port changes then the customer can update their configuration via the Realtime Switch Info page for the relevant Voice Group in the Clearfly Portal.

Registered (Dynamic) SIP Trunks

Configuration, provisioning and troubleshooting of registered trunks is typically much simpler for both the customer and Clearfly. With a registered SIP trunk, the customer’s phone system actually registers itself with Clearfly’s switch via a username and password provided by Clearfly during initial provisioning. If the customer changes ISPs or firewall configurations or something similar that results in an IP address change at the customer’s site, no changes should have to be made by the customer or Clearfly to restore phone service and the phone system should re-register as soon as Internet connectivity is restored. Please note that Clearfly only supports a single registration for each SIP trunk, regardless of the number of concurrent calls allowed on the trunk. Multiple registrations using the same credentials will result in erratic behavior and unreliable call completion.

SIP Subscribers

Unlike SIP trunks, SIP subscribers allow a separate registration per individual phone number. This type of service is typically used when a customer wants to directly connect an IP phone or an ATA to Clearfly’s network. Subscribers do not support direct-inward dialing (DIDs) or the ability to specify a calling number (ANI).

Feature Comparison

Feature Analog (FXS) CAS/RBS T1 ISDN PRI SIP Trunk (Static) SIP Trunk (Registered) SIP Subscriber
Static IP Required Yes1 Yes1 Yes1 Yes No No
DIDs Supported No Yes Yes Yes Yes No
Unconditional Call Forwarding Yes Yes Yes Yes Yes Yes
Unavailable Call Forwarding Yes Yes Yes Yes Yes Yes
Busy Call Forwarding Yes No No No No Yes
Delay Call Forwarding Yes No No No No Yes
Call Hunting / Hunt Group Yes No No No No Yes
Dialtone Provided by Clearfly Yes Optional No2 No2 No2 No2
Calling Number Control Portal Only Phone System3 Phone System3 Phone System3 Phone System3 Portal Only
1 Static IPs are necessary in order for Clearfly to manage & maintain the SIP->Legacy gateway
2 See our FAQ for more information
3 Clearfly will accept any valid NANP number as the calling number. A calling number can be configured in Clearfly's switch, but it will override the calling number for ALL outbound calls on the trunk.