this happens because there is a conflict in port number between the sip server port number and the default port number of sipend point.
to solve this issue you need to change the range of sip port numbers in the SipEndpoint.cfg file in the sipend point folder, specifically these two options:
<setting name="sip_port_min" value="5080"/>
<setting name="sip_port_max" value="5081"/>
make the range doesn't include the sip server port number.
up there i made mine a range of two unused ports only, but it is up to you to decide the value and the width of the ranges.