Fixed VoIP and Non-Fixed VoIP: Know the difference

VoIP has become commonplace in the commercial and personal world in recent times.   Businesses, large and small, have taken to VoIP because of the cost and business benefits it brings.   Simply put, it avoids the costs associated with PSTN connections by routing communications over the Internet.  This allows VoIP service providers to bundle in other features such as free or low-cost long-distance and international calls.

In addition to providing on-premises VoIP calls, the digital format of VoIP provides connectivity for WiFi-enabled smart devices such as smartphones and tablets.  Remote and home working have increased significantly because of the Covid epidemic, and VoIP over public and private WiFi networks and VPN connections are rapidly becoming the norm.


VoIP allows organisations and individuals to bypass the traditional PSTN infrastructure, thereby markedly reducing communications costs and providing additional business benefits at zero or meagre cost.   As always there are drawbacks, for example, in the US no access to emergency services like 411 and 911.

Please note that VoIP systems sometimes use PSTN services to complete calls, especially if the call terminates on the PSTN.   VoIP handsets are the usual desktop equipment to make and receive calls, but Analogue Terminal Adapters (ATA) can be used to connect legacy handsets to a VoIP network.  However, like some cell phone packages, the ATA may be tied to a specific service provider.

There are two types of VoIP systems, Non-Fixed and Fixed:

  • Fixed – associated with a fixed physical address.
  • Non-fixed – not tied to a specific address.

As one would expect, there are advantages and disadvantages to each, and choosing the correct profile will depend on individual business needs.

Fixed versus non-Fixed

Fixed VoIP vs Non Fixed

Fixed VoIP systems link to a specific physical location, non-fixed do not.  Most VoIP service providers offer both configurations.   The service provider has a broadband connection with the PSTN.

In short, fixed VoIP is one or more internet phone lines provided by the service provider.  There is an account manager for fixed VoIP, and a physical location attached to the VoIP dedicated line.   The physical address is needed to activate the service.   Usually, they will also support SIP connections through a SIP trunk as an alternate connection.

Because of the PSTN connection, there is usually a fixed monthly cost, perhaps higher than that of a non-fixed link.

Non-fixed VoIP numbers are, in essence, virtual phone numbers issued by free services such as Skype, WhatsApp and the like. Because they are not tied to a location, they can be used from multiple locations, provided an Internet connection is available.   

Non-Fixed numbers provide unrivalled connectivity for Global Communications, facilitating low-cost collaboration between geographically separate teams using voice and video.

They are also often used to provide local numbers that auto-forward to a central number, allowing callers to reach you at the cost of a local call.

VoIP Phone

  • Security. Fixed VoIP numbers are usually traceable, spammers and other criminals tend to use non-fixed numbers to allow them to hide where they are calling from.   Fraudsters often use a non-fixed number to place an order with a stolen or cloned credit card, secure in the knowledge that they are virtually untraceable.

    Fraud prevention is becoming a major item in VoIP security.  Being able to differentiate between fixed and non-fixed numbers and using tools for phone number validation and reverse lookup is more and more common.  Some organisations limit incoming calls to known numbers held in the corporate contact list.

  • International Access. Non-Fixed numbers by being virtual numbers and not tied to a specific location can be used from anywhere to anywhere, as opposed to fixed numbers that are more restricted in international usage because of the additional charges.

    Many common messaging and collaborative applications such as Skype and Google Voice use non-fixed numbers to provide location-agnostic collaborative communications.

  • Emergency calls.  Non-fixed  virtual numbers are not tied to a physical location and can have unreliable caller-id information, the emergency services are first, unsure of where the call originates and who from, and secondly to confirm the address to send first responders.

  • Regulatory Compliance.  Many jurisdictions have legislation around spam and unsolicited calling in general.  Most support an opt-out list function.  The unreliable call-id information of non-fixed numbers can make it difficult to see if a caller is blocked.  In the US there is some confusion in the legal community as to whether VoIP numbers are subject to consumer protection laws.