a ring group allows you to link multiple phones for ringing. The method of ringing an be all at once, sequential, or combination. The DN used for the ring group is the PRIME line of the set and all features associated with a prime line are available. You cannot route a ring group to VM or associate a message key
A multicall key can be associated with a prime line or be completely non prime. It will either ring or not ring. several features typically available to prime lines are not available. e.g. remote call pickup, remote hold retrieve.
You can route a multicall to vm and associate a message key.
suffice it to say they are diiferent enough to not be interchangable. You need to know exactly what you want to accomplish and choose the best app for the job.
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Occam's Razor - All things being equal, the simplest solution is the right one.