If you enforce IP range restrictions, you should add these IPs to the safelist. These are egress IPs for ScreenMeet's various global regions from where calls will originate to the SFDC REST API:
13.209.25.139
13.209.37.67
18.162.119.69
18.162.220.191
18.189.102.45
18.224.205.104
157.175.22.162
157.175.25.25
54.203.112.48
35.175.79.220
13.233.63.88
18.136.32.15
13.237.249.155
52.199.226.147
18.197.215.150
18.184.91.54
18.202.156.6
18.228.167.44