This question recently came in and I thought it would be good to go ahead and provide an answer here in the forum. This can happen if the SIP message exceeds the maximum MTU size (the default is 1500 bytes). This is required by RFC 3261. WebSphere does provide a way to change the maximum MTU size where this switch to TCP will occur. The reasons for doing this is that packets that exceed the MTU size increase the odds that the network will fragment the datagram which increases the odds that the packet will get dropped for some reason. My assumption is that the switch to TCP is considered more reliable.
The custom property at the SIP container for changing this is javax.sip.PATH_MTU. Here is the link in the Info Center where you can find all the SIP container custom properties: