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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
My first answer was https://silence.im/. However it seems to be semi abandoned and unmaintained.
The other option is https://jami.net/. It seems to have a phone option and could potentially replace your current system.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.