I'm facing similar issue but with SIP trunk to registering, Im using Yearstar PBX thats self hosted in the company's prem. Also testing using MicroSIP from an external network. the pattern i've noticed now is that livekit telephony works fine on some days but then all of a sudden it stop working. SIP trunk to registering with my own livekit SIP URI. microSIP not able to connect the call either. Only explaination I could think off after going through almost 2 weeks of this is that livekit SIP server is having connection issue but I really hope im wrong. my livekit trunk json:

{
  "sipTrunkId": "ST_GZiiMCbuyV7E",
  "name": "Yeastar PBX Inbound - test agent",
  "numbers": [
    "911"
  ],
  "allowedAddresses": [
    "0.0.0.0/0"
  ],
  "ringingTimeout": "60s",
  "maxCallDuration": "3600s"
}

dispatch rule:

{
  "sipDispatchRuleId": "SDR_KvRj2XAEZP92",
  "rule": {
    "dispatchRuleIndividual": {
      "roomPrefix": "sip-call-"
    }
  },
  "trunkIds": [
    "ST_GZiiMCbuyV7E"
  ],
  "name": "dispatch for test agent",
  "roomConfig": {
    "agents": [
      {
        "agentName": "ceo-assistant"
      }
    ]
  }
}

project id: p_4nqbraso3eq I even got the subscription thinking I might be limited by usage but that didnt fix it either.