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Remember that the problem may be at their end. In every phone (or fax) problem, there are three main parts: the originator or sender (at one end), the transmission line and facility between the two ends, and the receiver (at the other end.)
If the callers from U.S.V.I. are having trouble sending, there may be two things wrong: that the region is overloaded and switching/routing is inadequate, or that local towers or 'stations' may have dead zones (e.g. behind mountains, frequent in that region) or zones of poor coverage.