I know it's a little late, but Siemens HiPath is a very good product. Compares with NEC and Nortel. Not too expensive and very good customer support. Here's a quick view if you want to check them out.....The "First Tennessee Bank" and "Skyport" would be interesting to you. Good Luck!!

https://www.usaphone.com/productsservices_siemens.html