You should be seeing SIP messages from Linphone.
if you are not seeing any SIP message from Linphone then perhaps something else on that Linphone system is interfering with the Linphone operation or the IP packet sending.
Do you perhaps still have SJPhone installed on that system? You should only have one SIP application installed on system. If you have multiple different applications installed then the chances are they will be interfering with each other.
Was anything else installed on that Linphone machine apart from Windows+WireShark+Linphone?
Was anything else installed on the SJPhone machine apart from Windows+WireShark+SJPhone?
Please wee screenshot of Linphone we just installed on a system here which is placing an outgoing direct IP call. Nothing is answering that call, but you can see that SIP messages are captured in WireShark.
Sounds like you had better success with SJPhone, perhaps its a better idea to go back to SJPhone (uninstall Linphone first) and try to work out what could be blocking the RTP traffic from reaching VoiceGuide.
Suggest running WireShark on both machines at same time when placing a call into system to better see where the RTP traffic is blocked.