The overlap dialing feature in chan_sip allows chan_sip
to report to a device that the number that has been dialed
is incomplete and more digits are required. If this
functionality is used with a device that has performed
username/password authentication RTP resources are leaked.
This occurs because the code fails to release the old RTP
resources before allocating new ones in this scenario.
If all resources are used then RTP port exhaustion will
occur and no RTP sessions are able to be set up.
If overlap dialing support is not needed the "allowoverlap"
option can be set to no. This will stop any usage of the
scenario which causes the resource exhaustion.