Hello, Jean !!
I was faced same problem with Galaxy S4 and Galaxy S3 using Cisco Jabber Voice on VPN Connection and solved it 2 weeks ago.
I suggest that you have to check a firewall running VPN and disable the inspection of voice protocol such as "inspect h322 h225, inspect h323 ras, inspect sip, and inspect skinny" if you are using Cisco ASA Firewall.
HTH
Hank.