PBXes Causes Softphone to Return Busy

I am seeing this situation – testing with different Android softphones. PBXes rings my softphone extension. Because of network delay, the softphone does not reply within (approximately?) 5 seconds. After waiting 5 seconds, PBXes.org resends the ring. At the softphone end, the softphone sees two (2) back-to-back rings, within a couple of milliseconds. It is handling the first ring when the second ring arrives. The softphone correctly replies busy to the second ring – which causes PBXes to hang up on the extension.

Can PBXes fix this problem? In the interim, what is a workaround?

BTW, if I call the softphone from a different VoIP service, such as voxalot.com, I do not see the problem -- there are not two back-to-back calls.

Thanks,

George