VoIP, PBX and unified communications: replacing an enterprise phone system

VoIP, PBX, UCaaS and CCaaS: what each term covers

FCC rules define interconnected VoIP as a service that carries real-time, two-way voice over a broadband connection, uses IP-compatible equipment, and lets users generally receive calls from and place calls to the public switched telephone network (PSTN).⁶

The same rules treat a PBX as one kind of multi-line telephone system (MLTS). The definition also covers Centrex, VoIP, hybrid and key telephone systems, whether they run in the network or on the premises.⁷

NiCE, a contact center vendor, draws the UCaaS and CCaaS line by audience. UCaaS is built for communication among employees. CCaaS is built for customer-facing interactions at scale.^8,9

By NiCE's account, a UCaaS platform lacks what a contact center needs: automatic call distribution, queue management, workforce management, quality management, interaction analytics and omnichannel routing.³

PBX replacement: on-premises system, UCaaS or CCaaS

In its 2019 order implementing Kari's Law, the FCC found that the enterprise market had already seen significant migration away from traditional MLTS and toward IP-based and cloud-based systems.¹⁰

Subscription Edition can only be installed as an in-place upgrade of Skype for Business Server 2019.¹³

RingCentral lists its RingCX contact center separately from its RingEX phone plans, starting at $65 per user.¹⁴

Teams Phone: Calling Plan vs Operator Connect vs Direct Routing

Every Teams Phone user needs a Teams Phone license and a PSTN connection from a provider. Microsoft offers four ways to connect, and one tenant can use several of them for different users, locations and business requirements.^4,15

Microsoft's own example of a mixed design puts mobile workers on Teams Phone Mobile, knowledge workers on Operator Connect and workers in remote regions on Direct Routing.²²

Direct Routing is the route for keeping existing carriers or connecting third-party telephony systems. It runs through a customer-provided SBC, and the SBC's name must belong to a domain verified in the Microsoft 365 tenant.^23,24

UCaaS and CCaaS pricing: published per-user prices

Microsoft announced on November 12, 2024 new Teams Phone prices from April 1, 2025. It was the first price change for the standalone plans since they launched. Existing customers keep their current pricing until their next renewal.^25,26

Customers who get Teams Phone through Microsoft 365 E5 or Office 365 E5 on annual billing saw no price change in that announcement.³⁷

Contact center platforms charge in different units from UC seats. Twilio Flex also offers a $35 fee per monthly active user plus metered usage. Amazon Connect Customer charges per minute and per message.^35,38

E911 rules for VoIP and PBX: Kari's Law and RAY BAUM's Act

Kari's Law requires direct 911 dialing and on-site notification on multi-line telephone systems. Section 506 of RAY BAUM's Act required the FCC to consider rules so that a dispatchable location goes with every 911 call. The FCC's 2019 order applies Kari's Law to legacy systems and to IP-based and cloud-based services used by businesses, campuses and hotels.^5,39,40

• Direct dialing. Whoever installs, manages or operates an MLTS must configure it so a user can dial 911 from any station with no prefix such as 9.⁴¹
• Notification. The system must alert a central location at the facility, or another person or organization, when it can do so without new hardware or software. The alert must not delay the 911 call.⁴²
• Dispatchable location. The validated street address of the caller, plus details such as suite or apartment needed to find them.⁴³
• Deadlines. Fixed on-premises phones had to provide automated dispatchable location by January 6, 2021. Off-premises devices had until January 6, 2022, with a manually updated or best available location where automation is not technically feasible.^44,45
• Scope. Kari's Law applies to systems manufactured or brought into use after February 16, 2020.⁴⁶

Teams meets these rules through dynamic emergency calling. Admins map network elements to emergency addresses, and the client sends the matched location with the call. Calling Plans, Operator Connect and Teams Phone Mobile partners route these calls for users in the US and Canada. A Direct Routing customer must also connect an Emergency Routing Service provider or configure the SBC for ELIN.^47,48,49

STIR/SHAKEN and number porting when you change providers

FCC rules required voice service providers to fully implement the STIR/SHAKEN caller ID authentication framework in their IP networks by June 30, 2021, with set exceptions. A provider authenticates caller ID on the SIP calls it originates and hands to another provider.^53,54

A provider may use a third-party authentication service. It must still make every attestation-level decision, have calls signed with its own certificate and keep the agreement in writing.⁵⁵

Carriers the FCC requires to port numbers must complete a simple port within one business day, unless the new provider or the customer asks for longer. A non-simple port gets four business days. Ports involving interconnected VoIP count as intermodal ports under the same rule.^56,57,58

A port into Teams needs a signed letter of authorization and the billing telephone number the current provider has on file. A wrong billing number or a freeze on the account gets the request rejected.^59,60

For Operator Connect, Microsoft lists porting support among the factors to compare between operators, alongside country coverage and calling plans.⁶¹

VoIP call quality requirements and where migrations go wrong

Failure modes named in Microsoft's documentation and FCC rules:

• VPN hairpinning. Teams traffic forced through a VPN can hairpin through the VPN device and reach a distant service front door, adding latency and jitter. Microsoft recommends a split-tunnel path that bypasses the VPN.^64,65
• UDP blocked. Teams falls back to TCP when UDP is unavailable, and TCP hurts performance. Some VPNs do not support UDP.^66,67
• No QoS. Microsoft says QoS should run on every segment of a managed network, even one with enough bandwidth.⁶⁸
• A 911 gap in the browser. Dynamic emergency calling and security desk notification do not work in the Teams web client. Microsoft's fix is a calling policy that blocks PSTN calls from the web client.⁶⁹
• The wrong number type. Teams Phone Mobile cannot carry auto attendant, call queue or audio conferencing numbers.²⁰
• Broadband and power loss. Interconnected VoIP providers must warn every subscriber that 911 may be limited or unavailable after a broadband failure, a power loss or a move of the equipment.⁷⁰