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SIP vs VoIP: A Simple Guide to Similar but Different Terms

by Margaret Joy

SIP and VoIP are yet another pair of terms popularly used as synonyms when there's much more nuance to their true definitions. Here, we detail that nuance.

For terms that are used practically interchangeably in the world of business VoIP service, SIP and VoIP aren’t really synonyms. We’ll get into the details below, but as a general introduction, we’ll give you this: VoIP is the Internet-powered alternative to landline phone calls; SIP is an Internet protocol that allows communications to function. For an easy way to remember which is which, look no farther than our name! Voice over IP telephony runs on SIP protocol. On SIP. OnSIP. 

A quick glossary:

  • VoIP: Voice over IP
  • SIP: Session Initiation Protocol
  • PSTN: Public Switched Telephone Network
  • PBX: Private Branch Exchange

VoIP, VoIP With SIP, or SIP?

VoIP and SIP are tightly interwoven but not mutually inclusive. VoIP doesn’t need SIP to function as long as you’re just using voice communications. There are a few different Internet protocols that facilitate voice communications; SIP is simply the one that’s most widely used. SIP also enables text and video. Any business looking for an all-encompassing UCaaS provider should look for VoIP with SIP. If you just need Internet-based voice and no connection to the PSTN (aka regular landline numbers), VoIP alone should work for you. 

Confused about the whole “protocol” aspect? No worries, we’ve got you covered. You’re already familiar with many Internet protocols and their related technologies. Go click twice on the URL bar of this page. See how the second click makes the “https://” part of the URL show up? HTTPS is the protocol that enables data transfer, namely web page content. 

If you ever had to set up an email on your phone before everyone defaulted to Gmail, you likely had to plug in some settings labeled IMAP, POP3, or SMTP. These are email protocols: the proverbial Internet highway for sending and receiving email messages. SIP is the real-time communications protocol for video and voice calls, as well as text.

Using VoIP alone fairly limits your options. SIP allows you to scale your communications needs with your business. Most importantly, SIP can interact with other protocols. In plain terms, this means your VoIP system won’t hit any snags trying to connect to other systems, even if they use different protocols. 

For a full primer on Voice over IP telephony from when it started to how it works, check out our exhaustive guide to business VoIP.  

SIP Trunking

Businesses using a PBX phone system need to connect to the PSTN to make calls. This is done through a SIP trunk. If you’re using a fully hosted VoIP system like OnSIP, trunking is built in. For on-premises or hybrid systems (i.e., an on-premises system at a business with remote employees), you’d likely need an additional SIP trunking system. Luckily for OnSIP customers, we worked out a way to incorporate SIP trunking into our service no matter if you’re using hosted or an on-premises PBX. 

Trunks existed with regular landline telephones; SIP trunking is just the Internet version of trunking. A trunk in traditional telephony is a single line used by many people. Trunking is a way to share access by grouping people on a single line, rather than providing individual lines. VoIP trunking is quite similar—you can have fewer “lines” than people, just without the physical copper wire telephone line. 

Long story short, you likely don’t need to worry about SIP trunking because quality UCaaS providers like OnSIP already have it incorporated. 

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