I ended up in building the solution myself. 

here I publish some work-in-progress details if you're interested:
http://txlab.wordpress.com/tag/pbx/

Also I'll be glad to use my new knowledge for someone's project if there's a 
need for a similar solution.





>________________________________
>From: Stanislav Sinyagin <[email protected]>
>To: "[email protected]" <[email protected]>
>Sent: Tuesday, September 6, 2011 11:04 AM
>Subject: [swinog] looking for a virtual PBX provider
>
>
>hi all,
>
>
>I'm building a technical infrastructure for a new on-call support service. 
>This will require a virtual PBX where the support engineers would login and 
>manage the hotline redirection to their personal PSTN or SIP destinations.
>
>
>Those interested in providing such a service, please contact me off-the-list, 
>and I'll send you detailed technical requirements.
>
>
>
>
>cheers,
>stan
>
>
>
>
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>[email protected]
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>
>
>
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