As the OP, I want to point out that after several years and trying several different voip software programs, I can say that this problem is not specific to zoiper.
The problem is devastating however as it does appear to cancel voicemail from picking up whenever the problem occurs, which results in lost business for me. It seems that the problem only occurs when the phone has been sleeping for a while, thus making it nearly impossible to test different configurations on a practical level.
I have worked endlessly with my voip carrier to try different settings and no matter what I do, I cannot bring this to a satisfactory level of reliability.
I'm pretty sure there is a major issue with the LG F60 supporting any SIP software. (It's also known as the LG-D393 )
I have also noticed that this phone requires to be rebooted from time to time as the audio driver seems to "Crash" from time to time, resulting in most sounds not working in addition to countless other problems.
To resolve this problem I will likely buy another phone, but I am not sure which models work well with sip.