In some cases, a NEC PBX may drop calls in progress after they have been connected for almost exactly 30 seconds.
This issue is known to affect a broad range of models, including SV8100 , SV9100 and SL1100.
The resolution is to enable "Pendantic Mode SIP" on the device, which can be done by following the instructions below.
Instructions:

Release Notes:

Enable the Fix:
Sip_Cust_Script_84-34-09=1_Enable_Pedantic.pcs
Disable the Fix:
Sip_Cust_Script_84-34-09=0_Disable_Pedantic.pcs