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If another iPhone can work in the same vicinity, then your 'signal' is available for sure.
But check in your network settings, under Settings on your own iPhone:
- make sure that Fast Data Connection or 2G/3G/4G/5G data access is enabled.
- or if you use local WiFi, make sure that is enabled.
Then (if you use local WiFi) be sure to Connect your WiFi.
Then, after a few more seconds, once you're sure you're connected, try a call again.
(What phone service do you use? in case it matters.)