SIP trunks are the modern version of ISDN phone lines that route calls over your Internet connection rather than a dedicated digital or analogue phone line such as an ISDN or PSTN.
M12 Solutions have been providing SIP trunking service since 2007, and today, we almost exclusively provide SIP trunks and hardly ever provision new ISDN services.
SIP trunking has come a long way since 2007. Most of the early complaints about SIP trunking were to do with poor call quality and dropped calls. These problems were generally a symptom of poor Internet connectivity. Nowadays, connectivity is much better, and improvements in codecs and better monitoring means that support calls due to poor quality speech issues have become a rarity.