depending on what you're using it for, Pushover [0] might fit the bill. it's what I use for my home monitoring setup - low-priority alerts just go to an email folder, but a high-priority alert (such as a water leak sensor firing) will get pushed to my phone.
it's a "no frills, in a good way" type of product. dirt-simple API with straightforward pricing ($5 lifetime subscription) and limits so generous I've never worried about hitting them (10k messages/month).
I was actually using Pushover before I switched to OpsGenie. I typically have my phone set to do not disturb overnight and Pushover didn't have a "critical alerts" option to punch through - OpsGenie did. I may revisit it since I can dredge up the integration code from my Git repo.
it's a "no frills, in a good way" type of product. dirt-simple API with straightforward pricing ($5 lifetime subscription) and limits so generous I've never worried about hitting them (10k messages/month).
0: https://pushover.net/