Endpoint (your phone) --> Your gateway (for example, Asterisk) --> Carrier (a CLEC or ILEC) --> Terminating Carrier --> Endpoint (landline phone)
If you go up a layer, to the IP level, you'll have a number of routers and network links that make up the connection from your endpoint to your gateway. Each one of these is an additional potential point of failure.
If your testing shows that communication between your gateway has no problems, then I'd suspect the problem to either be with your Asterisk server or between your server and the carrier. If you don't control the gateway the latter will be challenging if not impossible to diagnose for sure.--
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