Just to explain shaun’s hint: the firewall of your voip board may block logins from devices with ip-addresses from outside of green network mask.
That’s why matching devices on firewall green instead of blue may be better.
Does your voip board have a mac filter? Your phone may change mac cause it feels to be in a new wlan.
Android does by default react this way, iphone too.
Normally this should only cause trouble with sip and not rtp. So check rtp-ports both ways. Normally rtp is a range.
Just look at Print from Android phone to LAN Printer