For restoring phone passwords using COBRAS, anything higher than CUC version 7.1.3 would do. Thus, it should work for CUC 8.6 also.
Please refer to the following excerpt from the COBRAS help file:
http://www.ciscounitytools.com/Applications/General/COBRAS/Help/COBRAS_Briefcase/COBRAS_Briefcase.htm#_Toc276653819
"IMPORTANT NOTE
: Phone passwords (PINs) from Unity 7.0, Unity Connection 1.2 and Unity Connection 7.x backups will NOT be restored when importing those users into Connection 7.x systems that are earlier than 7.1(3). This has to do with a missing functionality in the credentials management system in the 7.0 releases. This was addressed in the 7.1(3) release of Connection at which point COBRAS takes advantage of it and provides PIN support for all three versions. If at all possible please try and upgrade to 7.1(3) or later prior to importing data for this reason."
HTH!
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Regards,
Saurabh Agnihotri