Connecting the Cisco IOS Voice Gateway to CUCM via SIP has been the preferred way to do it in the past couple of years.
The slowly dying H323 protocol (ISDN based) is not being developed anymore while SIP (HTTP based) became the industry standard for VoIP. So whenever PSTN connection is implemented via an IOS Voice Gateway, the choice should be really between SIP & MGCP.
There is no way around it, finding the SIP call flow is the first thing you have to do when you are facing a SIP call failure.
Why? Easy. Because first of all, you have to understand whether this is a call routing problem or signaling/media compatibility issue. SIP call flow helps you understand just that, and in a lot of cases, you can pinpoint the problem just from looking at the SIP call flow.