Andrews & Arnold has a base of thousands of SIP customers. These range from individuals with a single number (perhaps replacing a now-defunct landline) through to businesses with large blocks of DDIs, and one phone handset on every desk.
Over many years, and after a series of implementations of both OpenSource and developed-in-house solutions on Linux (starting with Asterisk) it was eventually felt that a deployment of core SIP functionality on FireBrick would be beneficial.
Initially the solution entailed a pair of FB2700s talking to back-end RADIUS servers. There is a plan to replaced with two pairs of FB2900s, each pair located in its own separate datacentre. The system handles thousands of SIP accounts with peaks of hundreds of concurrent ongoing calls. It is extremely reliable.
Although perhaps not as feature-rich as some more common solutions, it fits the A&A customer base well (who often operate their own local PBX between the phones and the SIP accounts), and the high reliability does mean that VoIP is really 'fit and forget' in much the way ISDN or landline based PBXs were in days gone by.