SIP trunks can work in the cloud - but in what way?  This article explores the ways this can potentially work

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Voice over IP, or VoIP is a commonly used telephony system that companies today prefer to deploy due to its ease of use and compatibility with hardware that is more readily accessible by SMB's looking to host their own system but don't have the enterprise budget to spend.  

One integral part of this solution involves SIP trunking which provides the pathway in how people communicate over VoIP.  SIP trunks take on the traditional role where DID's or block of phone numbers get assigned to a SIP trunk.  These trunks run over traditional Internet feeds which in turn allows a company to either deploy dedicated links or to "share" the same link to save out on costs.  

So, the question arises - which can be used in the public cloud?  The short answer is "both" but comes with a big caveat.  A traditional SIP truck is tied to a ISP which usually means its a dedicated line that has to be dropped into in this case a cloud provider's data center.  If that is something they can accommodate, then great.  But, for most vendors, the second option is usually more appealing.  This has a few things going for it:

  1. It's not tied to an ISP and can share any valid ISP feed the cloud vendor's using 
  2. Phone numbers can be added or subtracted and is only limited by the vendor's policies
  3. Can usually use any IP-PBX software which can be ran on any cloud server
  4. Phone targets do not need to connect to the datacenter physically and just need a fast enough ISP at their location to allow for the phone traffic to be sent and received by their location

The biggest part here is to be sure that the SIP trunk you pick can be done without buying an attached Internet line.  Keep that in mind if you're looking to host your VoIP solution from the cloud.  Just search for "SIP trunk vendors" and see which fits your actual situation!