I've been using a Grandstream 752 for awhile now and it's always worked fine, however at some point in the last month or two (or three, it rarely gets used for outgoing calls) outgoing calls, and only outgoing calls, are failing. At the same time calls from a soft-phone work OK. From a SIP packet capture both the Grandstream and soft-phone go through the usual SIP INVITE/Unauthorised/ACK/INVITE challenge/response flow, at which point the soft-phone continues to Trying and the Grandstream retries the INVITE three times before failing the call.



I've been going through it with the ISP and they're pretty good with support but we're not really converging on a solution so thought I'd ask here in case the situation rings any bells with anyone. The weird thing is that registration works fine, incoming calls are fine, it's only outgoing calls that are failing, after working OK until recently.