All outgoing calls on my Samsung S4 Mini terminate with a Forbidden (403) message. There is no fault on my SIP provider's side because I can make calls from the Zoiper Windows desktop version using the same account settings and details.
Some history: I have struggled for 2 days trying to get this account to register at work on a fibre circuit (despite opening all ports on my Draytek firewall) but there was no problem registering at home on conventional broadband. The physical IP phones were fine at work so I checked their registration settings and discovered they used an outbound proxy server. I used the same outbound proxy details on Zoiper on my Samsung and it registered immediately. Now I can't make calls.
Does anyone have any ideas?