SIP Trunking SIP trunking enables VOIP access for your SIP enabled PBX. SIP trunks are sold per call session and provide all the features of a PRI with the cost savings of VOIP. Direct Inward Dial (DID) Caller ID Automatic routing to external number if your PBX is offline Learn More! Legacy Voice Want to bring the cost savings of VOIP to your existing phone system? Clearfly can deliver service in various ways: PRI/ISDN (1 to 23 Channels, or combine up to 4 PRI/ISDN) Traditional analog lines (FXS) CAS/RBS - We can do old school CAS/RBS T1's if needed as well.