SIP Client vs VoIP Phone: What Is the Difference?

"VoIP phone" and "SIP client" describe overlapping things from different angles. VoIP phone is the broad term: any device or application that carries voice calls over an IP network. SIP client is more specific: any device or application that uses the Session Initiation Protocol to set up those calls. A SIP desk phone is both. A SIP softphone is a SIP client in software form. A calling app that uses a proprietary protocol is a VoIP phone but not a SIP client. In everyday use, the real decision is between a hardware desk phone and a software client on a computer or mobile device.

What does "VoIP phone" mean?

VoIP stands for Voice over Internet Protocol. A VoIP phone is anything that digitises your voice, packs it into IP packets and sends it across a network instead of a traditional analogue or digital telephone line. The term says nothing about which protocol does the work. It covers:

• Hardware desk phones with an Ethernet port.
• Analogue telephone adapters (ATAs) that connect an old analogue phone to a network.
• Softphone apps on computers, phones and tablets.
• Consumer calling apps with their own closed protocols.
• Conference phones and door intercoms with IP connections.

What does "SIP client" mean?

A SIP client, or SIP user agent, is any endpoint that implements the Session Initiation Protocol described in RFC 3261. It registers with a SIP server, sends INVITE requests to start calls, answers incoming INVITEs and exchanges audio over RTP as defined in RFC 3550. The term covers both hardware and software. A desk phone from a mainstream manufacturer is a SIP client. A softphone app is a SIP client. A PBX that places outbound calls to a trunk also acts as a SIP client in that moment.

Because SIP is an open standard, a SIP client from one vendor can normally register to a server from another. This interoperability is the main reason to prefer SIP equipment over proprietary systems: you are not locked to a single supplier for phones, software and server.

How do the terms overlap?

Think of two overlapping circles. The VoIP circle contains everything that calls over IP. The SIP circle contains everything that speaks SIP. Almost every SIP client is a VoIP phone, since SIP is used for voice over IP. Most VoIP desk phones sold today are SIP clients. The parts that do not overlap are:

• VoIP devices using other protocols, such as legacy H.323 or MGCP equipment, or consumer apps with closed protocols.
• SIP used for things other than voice, such as video sessions or instant messaging.

When someone asks "SIP client or VoIP phone?" they usually mean "software or hardware?". The rest of this guide addresses that question.

Hardware desk phone or software client: how do they compare?

Aspect	SIP desk phone	SIP softphone
Availability	Always on, rings independently of any computer	Rings only while the app runs and is registered
Mobility	Fixed to a desk and network port	Goes wherever the laptop, phone or tablet goes
Audio hardware	Built-in handset and speakerphone	Device microphone and speaker, or a headset
Cost	Purchase price per device	Often free or a small subscription per user
Setup	Web interface or provisioning server	Enter account details in the app
Power	Needs mains or Power over Ethernet	Uses the host device battery
Updates	Firmware from the manufacturer	App store or installer

When does a desk phone make more sense?

• Reception desks, shared areas and meeting rooms where no one owns a computer.
• Environments where the phone must work even when computers are off or rebooting.
• Users who prefer a physical handset and dedicated buttons for lines and speed dials.

When does a softphone make more sense?

• People who already spend the day at a computer with a headset.
• Remote and hybrid workers who move between locations.
• Mobile users who want their office extension on a personal phone.
• Small teams that want to avoid buying and maintaining hardware.

Do softphones handle inbound calls differently?

Yes, and this is the most important practical difference. A desk phone normally stays powered and registered around the clock, so the PBX can reach it at any time. A softphone is reachable only while the app is running and its registration is current. On a desktop, that means the computer must be awake and the app open or in the background. On a mobile phone, operating systems suspend background apps aggressively, so a softphone needs a push notification mechanism from its service to wake it for incoming calls. Not every softphone has one. For example, SIPdia on iPhone must be open to receive calls, and on macOS the Mac must be awake with the app running. Check this behaviour before relying on any softphone as your only inbound device. Why some softphones must stay open to receive calls explains the background.

Can you use both at once?

Most PBXs allow more than one device to register to one extension. Asterisk's PJSIP channel driver sets this with max_contacts on the AOR (address of record) that belongs to each endpoint, and PBX front ends such as FreePBX expose it in the extension settings. With two or more contacts allowed, an incoming call rings the desk phone and the softphone together, and whichever answers takes the call. This is the most common setup in offices that offer a softphone as a supplement rather than a replacement.

Does protocol support differ between hardware and software?

Both hardware and software SIP clients vary in what they support. Features to compare include:

• Transports: UDP, TCP and TLS. Hardware phones sometimes lag behind in TLS support or certificate handling.
• Media encryption: SRTP support and whether it can be required rather than optional.
• Codecs: Hardware phones often include G.722 and G.729. Software clients vary. SIPdia, for example, supports Opus and G.711 only.
• NAT traversal: STUN and ICE support matter for remote users.
• Multiple accounts: Many softphones register several accounts at once, which desk phones also do but with a fixed number of line keys.

Next steps

If a software client fits your needs, see what SIPdia offers on macOS and iPhone. For background on how any SIP client works, read What is a SIP softphone? and What is SIP?.