We had some very frustrating times while trying to use the new 20S units. (Four of us on a 10-day tour with mileages up to 750 per day). The GPSs (four of them, one for each rider) would constantly blank-out intercom communication. Sometimes intercom would come back . . . . . some times NOT. This particular problem is an issue every time the Zumo lady speaks, Zumo rain alerts, Zumo dealership nearby alerts, Zumo Sonic restaurants nearby, or when any of the four of us would tap a button on the Zumo. Any interruption by the Zumo would cause loss of Intercom. It happened dozens of times per day to each of the four of us. We called it "Being Zumo'ed".
The issue seems to be the priorities within the 20S. The phone has the highest priority (after Ambient).
In the manual's instructions, there are "Cases" for pairing to the Sena. In both Case #1 and Case #2, the Zumo 665 is paired as a phone.
So if the Sena thinks that the GPS is a phone, and phones have the highest priority, could it be that the GPS thus has the highest priority? If it does, this needs to be worked on. Many times, our group was in disarray when we tried to depart from a GPS route to go eat or get gas. Just when we needed intercom the most, the Zumo took priority and intercom was impossible.