@TeleFox we have a quite old (few years) PBX which has been used in the years adding new and new phone lines with the company expansion. last batch of additions was 6 months ago: 5 new seats with new phones and headsets. PBX is Aastra with proprietary digital phones.
the company bought a new erp last december, against my advice they also buyed a generic "VoIP-ERP integration package". we are now implementing the ERP and the sysadmin at the ERP consulting firm pointed out to me that the package actually is a proprietary VoIP PBX based on asterisk.
the solution involves something around 10 or 15 people (depending on the layout of the customer care dept), company has somethin like 40 phones bwteen DECT and digital deskphones.
so we can:throw away the new asterisk PBX (wasted 4k€), sorry 15 people throw away current Aastra PBX , redoing a lot of cabling and rebuying the entire equipement for 40 between wirede and DECT phones try to integrate the 2, especially considering than it is right to isolate the ERP asterisk thing as you would not to depend on an erp for your phone system (ok, redo VoIP wiring and equipement if you want but choose a different strategy!)
now both current PBX managing firm and the carrier have found a number potential solutions (basically a sip trunk between the 2 PBXes).
hope this clarifies!