Sometimes, the app rings but the call is already disconnected when you pick up. This is especially guaranteed to happen in the morning on the very first call of the day. Not something new though, I have seen such behavior on other VOIP apps, such as Acrobits and Talkatone.
There are quite some VOIP apps for Android, but most of them have another big problem: they prevent your phone from sleep - your phone runs hot and battery drains quickly. Acrobits (newer versions), CSipSimple and Bria are such examples.
Sipdroid is the only Android app that does not have those issues, because it is all written for native Android, all new from scratches. It really idles, allowing the phone to go into deep sleep and still reliably receive calls. You can add G.729 codec to it.
However, there is a 5-year-old bug in Sipdroid. It'll lose registration after a while, especially after a change in network status, e.g., switching between WiFi and 3G or an Internet interruption. After looking into the source code and many tests, I'm happy to report: I fixed this bug.
If you need a dependable VOIP on your phone, the only available choice is Sipdroid and a SIP server that's done right, such as KOTS VOIP. No extra battery drain, reliable calls in and out, period.