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MPLS connection required to transfer calls from one PBX to branch office phones?

One of my clients has a central office with a VoIP PBX system managed by a different company.  They have one branch office now.  Both locations are tied together using MPLS connections.  There is really no data that needs to be transfered, they do need the ability to transfer calls from the receptionist at the mail office location to the remote office by just using the remote office extention, say 401.  They now are starting up a second remote office and will have a dedicated T1 plus a cable company supplied internet connection.  Will they also need an additional MPLS connection at this location also to have the ability for the PBX to transfer the calls?  The remote offices do not utilize any of the computer or printer resources at the main office.  The VoIP provider is wanting to install the additional MPLS, but I am wanting to try and save the client the additional monthly charge for a internet connection that may not be needed.  Thanks for your help and advice.
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MPLS will definitely work, and the vendor wants it because it is reliable way to deliver calls with the ability to ensure quality of service (if everything is setup properly).  Of course, the phone system will also work if you connect the two sites with a dedicated private line (such as a T1) but that is typically even more expensive.

You can also establish a VPN between the two sites and route voice calls over it.  This will be less expensive, but call quality cannot be ensured because of the “best effort only” design of the Internet and potential contention on the Internet access lines at each site.  You can mitigate the latter by having a dedicated Internet link for the voice VPN, but you are subject to call quality impacts if the Internet link is shared (such as residential class cable modems) or aggregated (such as oversubscribed DSL) and you still can’t guarantee performance through the unwashed Internet.  You can increase your chances of acceptable performance if both office locations use the same ISP, because traffic should then flow between the two sites with fewer hops.

With the caveat of “no guarantee” and “let’s try it and find out” you could give the VPN link a go and still revert back to MPLS if you have to.

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Commented:
Thanks so much for a concise answer.  Have a great day.