SIP registration blocked via a IP

HI,
I have 2 openvpn servers - and 3 SIP servers (2 sip in my control and 3rd is a 3rd party).
using both openvpn I can register to a 3rd party SIP server via a dialer and make calls.
However when using my 1st openvpn to register to any of my SIP server, I cant.
Where as when using my 2nd openvpn to register to my both SIP servers- it works.

Summery:
SIP server A, B and C
Openvpn X and Y

Using 'X' I can register to A, B & C
Using 'Y' I can register to only A and not B and C

B & C are my VOIP servers, windows based and I have full access to them.
When using RDP to B and C via Y - it works.