Generally the answer would be YES. SIP trunks are cheaper than ISDN30e and ISDN2e line rental, and call costs are lower when utilising SIP trunks, including zero cost site-to-site calls. However what is sometimes overlooked is the internet connectivity to deliver your SIP calls. As mentioned in the SIP reliability question, to try and push SIP calls across a low-end data connection could have a disastrous impact on your business.