To make use of PSTN calling within Teams users must either have the Phone System add-on license for their O/M365 plan or a plan that incorporates Phone System such as E5. This is the same as it was for Skype for Business online however up until now Teams PSTN calling could only be achieved in a pure cloud model using Calling Plans for Office 365 – Telstra Calling. The issue here being Calling Plans are available only for certain territories ruling out much of the globe from PSTN capabilities within Teams. Calling plans can prove expensive especially for larger businesses as each user effectively leases a single trunk whereas with an on premise solution we may provide one trunk per ten users. It also presents no option for coexistence between the legacy on premise platforms increasing cost and risk during migrations, especially when it's a slow, phased migration with parallel run needs measured in years.
With Skype for Business we had a potential although less than ideal solution with the Cloud Connector Edition (CCE.) This later became an embedded option within the ASM modules of leading session border controller vendors Audiocodes and Ribbon (Sonus.) CCE allowed office 365 homed Skype for Business users to route their calls via local SBC appliances and either onto SIP services or interwork with existing PABX’s. Teams does not make use of CCE which is a good thing but there is also no option for Skype for Business to use Direct Routing. To be able to route Teams PSTN calls via local SIP services Direct Routing is required. Microsoft have established a SIP Proxy service within Office 365 that facilitates connection of the SIP trunk (bring your own SIP trunk as it used to be known.) To connect with the service a certified Session Border Controller is required, currently only models from Audiocodes and Ribbon are supported and it’s only a firmware update, so existing SBC’s can provide Direct Routing. This also includes models that incorporate CCE such as Ribbons CloudLink and Audiocodes CloudBond systems and that’s good news because we can now support Skype for Business online users and Teams users that both require on premise PSTN integration from the same appliance as shown below.
|Courtesy of Ribbon|
There are still key issues to address and media bypass is at the top of that list. With the current implementation all call media will route via Microsoft cloud based infrastructure meaning in most cases call legs will traverse the Internet limiting scope for end to end QoS and adding unnecessary hops to the call. However this feature, along with others, is on the horizon and should not be an issue for most considering adoption in the near future.