still having same issues
Im not sure what to suggest, as we have already made changes so the INVITEs from MicroSIP and Asterisk are very similar.
Possible approaches:
Ask the STC support to give more information about the error, such as the INVITE sent from their server to the upstream, as well as the 503 response (which may contain a Warning header or other supplemental information).
If you have a iPhone with a physical SIM (it’s too much work to do this with an eSIM), try testing that SIM in an Android device. It’s possible that the error is not directly related to iPhone, but to the number or account.
Capture traffic on the pfSense WAN interface, in case proxmox or some network element is modifying the request.
Take the INVITE sent by MicroSIP and send it from Asterisk, using ncat or similar. If that fails, there is likely some network component altering the request. If it succeeds, try sending a failing INVITE with ncat. Assuming that fails, we know that some header (or SDP difference) is triggering the failure and you can experiment by changing one header at a time until you see which causes the trouble. We can then determine if there is a settings change (or possibly a code change) that fixes the problem.
Other options:
Fail over to another trunking provider that doesn’t have the issue. Unfortunately, because of regulations, most A-Z providers have high prices to SA mobiles – $0.14/min. is typical.
Fail over to another service (landline, GSM gateway, etc)
If these are calls to your own mobiles, and it doesn’t fail when calling an iPhone on another SA carrier, consider porting to that carrier.
If these are calls to your own mobiles, consider forwarding to a SIP app instead of a mobile voice call.