SIP vs PRI VoIP pros and cons

I am evalulating VoIP solutions to sit on an existing MPLS network and need some help to determine the pros and cons of SIP vs PRI.  The MPLS is a Metro-E fiber at a corporate site, then T1 at 10 branch sites all within a 100 mile radius.  There would be approximately 20 phones at the corporate location and 7 to 10 at each branch site.

Our telco has pictched a SIP solution, which has a monthly fee associated with it depending upon the number of sessions we need.  I have read some reports that SIP is cheaper that PRI but not as stable?  Some of the phone systems we have looked at would have a box that resides at the corporate site - in the event of an corporate outage we would like calls to still like calls to go to and from the branch offices - I have also read that the SIP trunking configuration can handle this.  

Any feed back and information would be helpful.
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SIP does have clear advantages over PRI in two main areas:  ability to size capacity to exact needs while accomodating a wide range of numbers (such as local branch DID numbers), and the ability to automatically reroute calls in the event of an access loop failure.  
It is the hot topic in the industry and you can read up on many issues at the link below.  You may have to set up a free account, but you can browse for SIP trunks and get many articles and insights from experts. 
One caution is the comment that integration and reliability are not the equal to PRI yet.  The SIP alternate routing is a necessity since it is not as stable as PRI.  And some carriers are not fully compatible with some systems - for this reason and for security it is recommended the client install a Session Border Controller (SBC).  Even if the carrier suggests that it is not needed, the carrier SBC protects the carrier, not the customer.
It is not right for everyone, but we are converting several multi-site clients to SIP.

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And further to JRSCGI's comments you should contact your phone vendor for a compatibility document.  I've seen the following issues at sites when SIP is implemented:  loss of Caller ID, loss of MW indication across links (assuming single VM at corporate office), loss of ability to externally call forward.  The phone vendor/manufacturer should be able to provide you with a compatibility document for SIP with interoperatibliy testing specs for each SIP provider (of which there are 100's).  
We are staying with PRI's for now until the compatibility fleshes itself out and stabiliizes.  My PRI's never go down.  Of course, now that I've said that...
Many of the interoperability problem can be handled by an SBC - see this link for good info:
But it is true that universal compatibility is not a given and each carrier and equipment vendor have their lists of test results.
And I agree that right now ISDN PRI is more stable / reliable - many of our customers are waiting as well.
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djpriceAtlAuthor Commented:
Thank you both, this input is very helpful in aiding me to make a better educated decision.  I did confirm there is an SBC on the MPLS (Ingate box I believe).  I have read either great or bad things about SIP.  As long as there is QOS and telco support than it appears to work out well.  PRI may not be an option for us since the T1 routers are owned and managed by my company, the telco requires them to own and manage the routers (at an additional cost, of course) and this may not be feasible.
Where are you located as there may be different providers that can support this in a better fashion. Some carriers do not have a true IP Core network and that can also be a point of concern so ensure you are using a carrier that has thier own PSTN as well as thier own Soft Switches to manage the SIP Services. this way gettin SIP trunks from them have a level of guarantee and local support just like the PRI and POTS lines have. Many carriers that have CPE equipment already use SIP out to that equipment that simply converts it to a PRI, analog Lines, or Trunks at that point. So SIP from a carrier is really not an issue. It is the vast array of SIP platforms that are not really standardized at the IP-PBX level. Many manufacturers each have thier own interpretation of what "Flavor" of SIP makes the most sense. regardless SIP is as stable if not more so than PRI services if all components agree on the "SIP" formatting. Most SIP Trunk providers can also support E911 at the DID level so that you do not need gateway devices to grab local POTS lines at each location to make 911 work as expected.

Where is the business located? I can give you some carrier references.
djpriceAtlAuthor Commented:
The business is in the Atlanta GA area, all 12 locations are in the general metro area.  Any carrier references would be appreciated.
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