Grafana OnCall is an OSS alternative (with a cloud offering) that works great out of the box if you are using Grafana/Grafana Alerting for monitoring your systems and want to have a pager-like system with phone/SMS/telegram integrations + it's own app. Best of all, it's self-hostable as well, which keeps me completely in control of my infra.

https://grafana.com/products/oncall/

https://github.com/grafana/oncall