How to setup FORM filter for SIP's INVITE message properly?

Use information inside barckets inside From line contained by INVITE message. For example, this line in detailed decoding window presents as:

From: <sip:stgru2@172.21.0.25;user=phone>;tag=1c772495090

Enter the next combination in the From field at the Connections tab placed in a filter setting dialog for SIP protocol:

stgru2@172.21.0.25;user=phone