Keep a list of the calls your present setup handles badly. Perhaps customers reach voicemail while another employee is available, or staff working away from their desks cannot answer the main number. Those problems give you something specific to evaluate in a VoIP demonstration. If you have no such problem, the question may be whether replacement would reduce cost or avoid an approaching support issue.
The line, the handset and the phone system are different purchases
VoIP carries speech over an Internet Protocol network. It does not describe one particular handset or subscription. A phone system also needs to decide where calls ring, how employees transfer them and what happens after hours. A PBX is the equipment or software that performs those switching and routing functions; it can be part of an older installation or an IP-based arrangement.
A hosted service places those functions with a provider. Depending on the service, employees may use desk phones, computer software or mobile apps. Replacing the carrier line, replacing the office phone system and replacing every desk phone are therefore separate decisions. Have the proposal identify which of them it includes.
The reverse distinction matters too: keeping an analog handset does not establish that you still have a traditional copper line. AT&T Business Voice, for example, is an IP-based replacement designed to support existing analog equipment. Ask how the service behind the jack is delivered.
Source: Cisco: Small Business PBX — The Basics.
Source: AT&T: Business Voice for existing analog and utility lines.
What would staff do differently on Monday?
Bring a normal workday into the demonstration. Have someone answer, put the caller on hold, transfer to an unavailable coworker and recover the call. Repeat the exercise for the staff member who works elsewhere. The point is to see the actions your team would actually learn.
Features such as an auto receptionist, team call queues and mobile calling can help with these tasks. They are not uniformly included: Zoom's license documentation, for instance, separates capabilities by license type. Ask which proposed users receive each needed feature and whether its configuration is included.
An office with changing staffing or several work locations may benefit from easier call-routing changes. An office that already answers calls reliably at one location may gain much less. Training time, replacement hardware and new administrative duties belong in that comparison. There is no benefit in paying for a more elaborate call menu if customers simply need a person to answer.
Some lines should come out of the ordinary phone discussion
Before accepting a bundle, identify any number used by a fax machine, alarm, elevator or other special-purpose device. A salesperson's assurance that the service supports analog phones is not confirmation for every device connected to an analog port.
Purpose-built replacement services do exist, including the utility-line applications described by AT&T. Have the company responsible for the device or monitoring service confirm the exact proposed arrangement, its power requirements and any applicable approval requirements. Keep that decision separate from buying employee calling features.
For fax, decide whether the machine is actually needed. A document-based fax workflow may suit an office that already works with PDFs; a physical machine requires a supported connection and end-to-end testing. Do not move an alarm or other critical line simply because a general-purpose adapter has a matching socket.
Source: AT&T: Business Voice for existing analog and utility lines.
The change is not ready until the outage and emergency arrangements are clear
Have the provider describe a local power failure and a lost internet connection separately. Which phones stop? Which equipment needs battery power? What can staff use elsewhere? Zoom's 911 notice explicitly identifies power and connectivity failures as service limitations. Backup arrangements must cover the equipment and connection needed by the chosen service.
An existing line also needs examination: a cordless base, local PBX or adapter may depend on building power. Do not give either the current or proposed system credit for outage behavior that nobody has verified.
Emergency calling needs correct location handling, especially when a phone or app moves. Zoom's notice describes registered-location responsibilities and possible delays when location changes are processed. Obtain the new provider's setup and verification instructions for your workplace rather than making an unarranged test call to 911.
If the proposed features are useful but these details are unfinished, request a staged migration. Agree which ordinary voice numbers can move first, which services remain in place, who trains staff and what must be demonstrated before the old service is cancelled.
Source: Zoom Phone: 911 Customer Notification.
Source: FCC: Backup Power for Residential Voice Services (2015 technical background).