At my work, we just converted our 6 POTS lines over to SIP trunks. The primary reasoning behind it was cost. We've been paying $67/month for "Local Loop" (a centrex-type) line, and the price increased in February to $75 for each line. We had enough. Even a basic POTS line is $58/month, which also increased in price as well.
It's hard to justify that price when there are alternatives for a fraction of the price.
Telephone Reliability has always been a prioritiy for us. I went to our telephone provider first and asked about SIP lines. Obviously if they can provide relaible POTS service, I'd expect the same for SIP. They essentially told us that "Their SIP Service isn't oriented for small business. Unless we're converting multiple T1 lines, we won't find a cost savings with converting to SIP". I was shocked to hear this.
So I went to our ISP. We have a fiber + standby DSL connection to our ISP. They are also the LEC for about 15 rural towns. We were able to get SIP service from them for $20/month/line!
We have redundancy in place (fiber + standby data DSL + standby voice DSL), it's been just as reliable as our POTS lines. In addition, the calls are clearer, and we've been able to save $5300 a year by converting. Plus, since our voice and data are handled by the same ISP, our SIP traffic stays locally within the ISP's network.
I think the major telephone providers (up here in canada at least) are killing the POTS phone service simply by how much they charge. Why does an analog line cost $32, PLUS $8 for Caller ID (something the line cards probably already have in place whether it's enabled or not), PLUS another $2.50 for "touch-tone" service?
Up here, they're in the process of converting all POTS lines to be VDSL ready, as well as moving to fiber for all new builds. However, I really think they need to evaluate their basic POTS service and realize their competitors are KILLING them in the price. I bet a lot more people would stay if basic telephone service could be priced in the $10-15 range. I know it would never happen though.