SMSTunnel for Prometheus Alertmanager

Help me get Prometheus Alertmanager alerts as text messages from my own phone, with SMSTunnel.

About SMSTunnel (https://smstunnel.io): my own apps and tools hand text messages (SMS) to my SMSTunnel account, and my own Android phone sends them from my own number, on my own SIM and plan. The phone runs the SMSTunnel app from Google Play (https://play.google.com/store/apps/details?id=io.smstunnel.app), signed in to my account.

What I want: [describe what should send a text, to whom, and when]

How it works:
- No plugin. The easy way, on any Alertmanager version: an SMSTunnel webhook address (made in my dashboard with Alertmanager picked) as a webhook_configs receiver's url. With an API key instead, on Alertmanager 0.32 or newer: a receiver with "payload" and "http_config.http_headers" calls SMSTunnel's API.
- Webhook address way (any version):
route:
  receiver: sms
  group_by: ['alertname']
  group_wait: 30s
  group_interval: 5m
  repeat_interval: 4h

receivers:
  - name: sms
    webhook_configs:
      - url: https://smstunnel.io/api/v1/webhook/<token>
SMSTunnel reads Alertmanager's own JSON and texts "[FIRING] <alertname> - <summary>" (and "(+N more)" for groups) to the numbers saved in the webhook. Each notification of a group is one call.
- Without a key: an SMSTunnel webhook address, https://smstunnel.io/api/v1/webhook/<token>. I make it in my dashboard (https://dashboard.smstunnel.io): Apps → Webhooks → New webhook, pick the tool that will call it (Grafana, Uptime Kuma, Healthchecks, Gatus, Alertmanager, Odoo Online, or Something else), add 1 to 10 numbers (each gets its own text), optionally a text of the message that pulls fields out of the JSON with {{path}}, like {{alarm.name}}, then Create the webhook and copy the address. A GET with ?message=… or a POST with JSON (its message, text, title or body field) texts those numbers; the call never sets the number. Texts are cut at 480 characters. "Calls an hour" (20 unless I set another number, up to 500) counts calls, not texts: each call texts every number, and calls past it are refused until the hour is over. The address can be copied again under Address and setup; New address replaces it if it leaks. Scripts can make one with POST https://smstunnel.io/api/v1/webhooks and my sign-in token instead.
- API key way, config (0.32+):
route:
  receiver: sms-me
  group_by: ['alertname']
  group_wait: 30s
  group_interval: 5m
  repeat_interval: 4h

receivers:
  - name: sms-me
    webhook_configs:
      - url: https://smstunnel.io/api/v1/sms/send
        http_config:
          http_headers:
            X-API-Key:
              files: ['/etc/alertmanager/smstunnel.key']
        payload:
          to: '+1 415 555 0123'
          message: '[{{ .Status | toUpper }}] {{ .CommonLabels.alertname }} - {{ (index .Alerts 0).Annotations.summary }}'
The key file holds only the key (no newline). "secrets: ['<key>']" works too but keeps the key in the config.
- Careful with the API key way: Alertmanager reads every rendered payload value as YAML. A bare +14155550123 turns into a number without its +, and SMSTunnel then adds the phone's country code: a wrong number. Write the number with spaces, like '+1 415 555 0123' (SMSTunnel removes them). A text like "FIRING: x" can turn into a map: keep it starting with "[", as above. toJson doesn't help in 0.32–0.34 (its quotes stay in the value).
- One recipient per request: SMSTunnel sends one text to one number. For several people, one request (or one notification) per number. With a key, one webhook_configs entry per number; a webhook address texts up to 10 numbers itself.
- send_resolved is true by default for webhooks: resolved alerts text too. repeat_interval resends a firing alert (4h above).
- Test: curl -X POST http://localhost:9093/api/v2/alerts -H "Content-Type: application/json" -d '[{"labels": {"alertname": "SMSTest", "instance": "nas1"},
       "annotations": {"summary": "Test text from Alertmanager"}}]'
- Answers: 200 {"success": true, "messageId": "…"}; 401 wrong or deleted key; 403 the key lacks a permission ("code": "PERMISSION_REQUIRED"); 400 a missing, unknown or too long field; 429 too many requests (30 sends a minute, with Retry-After): wait and retry; without Retry-After, the plan's limit for the day or month is reached. If my phone is offline the text waits and goes out when it is back, within 3 days.
- The API key comes from my SMSTunnel dashboard (https://dashboard.smstunnel.io): Apps → Add an app → Create the key. It is shown only once and can send texts and check whether they went out.

Rules for your answer:
- Do not ask for my SMSTunnel password or API key and do not put a real key in your answer; I will not paste it in this chat.
- Phone numbers must be in international format (+ and the country code).
- Help me test it with my own number first.
- Only send texts people expect from me. No unsolicited texts.
- Help me route only the alerts that deserve a text to this receiver, and avoid floods (grouping, repeat_interval, inhibit rules).