[Users] empty response from mediaproxy

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Hi,

Recently, I found that there is the following error in the log.
error: use_media_proxy(): empty response from mediaproxy

The error will cause a looping problem last for 5 minutes (looping
time out).  The result is user failed to make call and the network
will be congested if serveral users having the same looping problem at
the same time.

What will cause that error?  Is it a bug for mediaproxy?

system configuration: openser 1.0.1 + mediaproxy 1.4.2

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