eSpace 7950 version: V100R001C30SPC200
eSpace U1910 version: V100R001C01SPC700
After check the topology in network of customer, find out they have UC1.1 topology.
The IP phones had UC2.0 as a network environment.
Change all the IP phones to UC1.1 network environment.
No more disconnections occur.
Check configuration on PBX U1910 about users and trunk protocol.
Check the configuration on IP phones eSpace 7950.
Always verify the topology according to the network environment configuration on IP phones.