Time was people would have SIP trunks into their business but always in tandem with an ISDN line. This showed an element of distrust in SIP. They liked the idea of the cost savings that VoIP could bring but had heard it was unreliable.
Even here at Timico we did the same. As a service provider our customer service function is mission critical. When we started our SIP ran over bonded ADSL so we had SIP trunks and ISDN. The funny thing was whenever there was a problem it was with the ISDN line. It was the SIP that always came to the rescue.