I am starting out with VOIP and configuring softphones. When we sign up to a SIP provider like FlowRoute we get the credentials for a SIP Proxy. Can't we just use the SIP registration details in a softphone like Zoiper ?

Why would we want to setup a PBX? I am talking for setting up a simple system to make calls from a desktop.

Why must there be a PBX in the middle ? Why setup a PBX and add the SIP details there and configure the softphone with Asterisk ? Why not add the SIP details in the softphone directly ?


The PBX is going to handle all of your endpoints, your hunt groups, your schedules, your auto-attendant, call flows/trees, recordings, etc.. Your SIP provider gives you phone lines. The PBX allows you to use those phone lines. A lot of SIP providers can also provide hosted PBX services, so they handle everything.

You could set up a PBX in your office for internal communication between your employees. But if you want to attach public phone numbers and allow incoming/outgoing phone calls, you'll need to add a SIP provider.


If you just want a simple phone, a VoIP equivalent of just renting a single phone line and plugging a dumb phone into it, you don't need the PBX, just a Sip Trunk.

If you want the features that the PBX brings to a traditional phone line on your VoIP line, you need a PBX for that too. Some Sip Trunk provides provide a few of these features at the host end but unless you take a "Hosted PBX" type service, (in which case you still HAVE the pbx, it's just at the host end!) you won't get most of them.

  • Hunt groups
  • Call Forwarding
  • Voicemail
  • Conference calls
  • Call Recording
  • to pick on just a few of the more common...

If you don't fancy Asterisk, and you don't want to pay, check out the "free" version of 3CX, windows based soft-PBX.


You absolutely can do this. Whether it's appropriate for your situation is a different matter.

Essentially, SIP trunks are like POTS lines or T1/PRI trunks. They are just dial tone. They can be used (and combined) in various ways depending on the business application.

Using that analogy: You could run a individual POTS lines to each desktop (i.e. associate your softphones with raw SIP trunks). This would get you phone service at that desktop / workstation.

If you have more than one desktop, you may want to call between them, transfer calls between them, or have them ring sequential in a hunt group when an inbound call to your company (or a specific department, maybe on an extension) comes in. This is where a PBX needs to exist somewhere.

The PBX can either be on-premise (e.g. Asterisk, Avaya IP Manager, etc.) or off-premise (e.g. hosted by your SIP provider).

Some SIP providers just provide the equivalent of "dialtone" -- just trunks. These can be connected directly to endpoint devices like hardware phones, soft phones, or on-premise PBXes.

Some SIP providers also provide hosted PBX offerings. In these cases the customer doesn't have an on-premise PBX, and their endpoints are configured to point at the provider's PBX.

  • So if I register to a SIP provider like Flowroute and only want to make calls to the PSTN via flowroute there is not need for the PBX right ? I can just download something like Zoiper and add a SIP account and make calls right ? Am I correct ?
    – Kramer786
    Sep 1 '16 at 7:39
  • I don't know about FlowRoute specifically, but yes generally ...at least for one endpoint device. Sep 2 '16 at 18:00

If your looking at VoIP line to replace a personal line you could use this as a very basic phone line, possible for a home. You would need to see if your provider could provide voicemail or call waiting.

For a office situation you will want the free inter-PBX calling. If everyone is registering to a 3rd party that 3rd party my charge you for those interoffice calls.

Their are several providers which can provide a whole PBX managed offsite, however if your office has a large concurrent call requirement you will need to make sure your location can meet the bandwidth requirements. The biggest benefit of a off site PBX would be if you have a large remote/home office user base. If a majority of your users are in one office a on site PBX may be beneficial.

If you looking to convert to VoIP also check what e911 regulation may govern your organization, and confirm your provider can comply with these.

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