- Focusing their attention on leveraging SIP to reduce the number of PSTN trunks; or
- Over-subscribing IP bandwidth to better size access trunks to corporate needs.
But there are two other potential ways SIP trunking can reduce costs that often get overlooked:
- Using SIP trunking to reduce mobile voice minute usage; and
- Reducing the number of trunks required to support fixed-mobile convergence.
SIP trunking vendors that also offer mobile wireless services can integrate the two offerings, enabling calls made to and from their mobile subscribers to ride over their SIP trunks, often without any per-minute costs for those calls. This setup means that the calls between enterprise telephones and mobile phones on the SIP trunking service provider's network don't count toward usage-based rate plans.
SIP trunking services bring single-reach numbers
Another rising problem that SIP trunking services can address is the need to tie up multiple trunks to forward calls to mobile devices. In architectures where the PBX resides on the enterprise premises, forwarding or simultaneously ringing in-bound calls to mobile devices ties up two PSTN trunks: one for the incoming call, and one for the outbound call to the employee's mobile device. Given the rapid rise of mobile devices, with many employees now carrying two or more mobile phones, enabling simultaneous ring can rapidly drive the need to add more PSTN trunks.
Marrying SIP trunking with service provider signaling infrastructure can solve this problem. A service provider that federates its call signaling systems with an enterprise PBX -- usually via the IP Multimedia Subsystem (IMS) -- can both send a call to the mobile device and the enterprise PBX by mapping mobile and desktop numbers together. Since the SIP trunking service provider is capturing and sending the call to both devices, the call doesn't first have to reach the enterprise PBX, which then places a second call back out to a mobile device. Extended even further, a user could potentially switch a call from an office phone to a mobile phone simply by signaling the carrier network to transfer the call -- which, again, saves an outbound port.
IMS federation between service provider and enterprise PBXs via SIP trunking opens up additional possibilities: A service provider can take calls made to a user's cell phone number and route them directly to the enterprise PBX, enabling single number reach, enterprise control over mobile phone calls and unified messaging. Service providers can offer the ability to respond to calls via SMS, allowing employees to send "'I'm on the phone, but I'll call you back" type texts to incoming calls.
Unfortunately, however, these latter and more innovative examples of fixed-mobile convergence are not yet widely available. Still, those responsible for evaluating and selecting SIP trunking services should discuss with their providers the potential to reduce costs by integrating mobile voice services with SIP trunking, and they should learn their carrier's plans for leveraging IMS to further integrate mobile devices and on-premises IP telephony platforms via SIP.