It states "This easy-to-manage platform supports up to 138 phones and 125 voice mailboxes..."
The base UC560 now supports 24 user licenses. But you can increase the numbers of licenses via adding increments of L-UC-PRO-8U= check out table 4 from the reference above.
Depending on your companies Cisco certifications, you could start looking at the Business Edition products.
For example BE3000 can cater for up to 300 IP phone users but you have to be at least Select certified to sell that product. But if you are a Cisco certified Partner you would know what you can sell and support.
You cannot really create an single autonomous phone system by adding two UC560's together. They will still be two autonomous phone systems.
The best thing for you to do is to upgrade to an ISR to support the numbers. This could mean that you’ll need new licenses and new phones depending on the model of phones you use with the UC500. The SBS phones are not supported on the ISR platform for example. Talk to Cisco maybe they’ll do you a deal on the licensing, and you may be able to trade in what you have for a new ISR.
You could run two UC’s. You will lose some functionality when calling between them, they’ll work separately of each other, e.g. phonebooks are separate. The second UC can make PSTN calls via the first UC500 and you’ll be able to transfer between them, although some or all of this may not be supported by Cisco.
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