zoiper community site の中に似た事象を見つけましたが、それによれば 最新版の zoiper BETA を使ったら収まったとの事例がありました。
I confirm that this issue is RESOLVED with the ZOIPER BETA (Zoiper v2.0.19, librairy rev. 2.8.12).
I had the exact same issue: after 10~20 hours, Zoiper (v1.40, library rev. 2.8.9) was remaining stuck in orange state "establishing connection" or red unable to connect. Forcing reconnection from the application was always unsuccessful. The only way I found was the same as upper her: quitting the application (also closing from Android interface) and launching again.
The new beta version works perfectly without interruption for 72+ hours.