The issue arises when the number of FXO required is more than the 12 supported by the UC560, or there more than 6 BRIs accordingly.
What is, based on your experience, the most cost effective solution to support more than 12 FXO or 6 BRI.
Please note that unfortunately a PRI is not an option.
The SPA400 will not help you as that product was only designed to work with the SPA9000 IP PBX. You might want to look at the SPA8800 that also provides four trunk lines. While we have not explicitly tested and so do not formally support integrating the UC500 and SPA8800 yet, we have done so for the SPA8000, and there is a common code base in use between the two platforms. As such, you should be able to make this work easily enough. The UC500/SPA8000 configuration note is located at https://supportforums.cisco.com/docs/DOC-9465/.