Rb2011us VoIP disconnect after ~ 20min

Hello guys,

Yesterday i bought 4 Linksys VoIP Phones, when i connect them to my MT RB2011us everything works, Phones properly registered to my SIP provider. After ~20 min i was unable to call anything even in my private network. After MT reboot, everything was back to normal for about 20min. Software and firmware ugraded to latest version

SIP Healper disabled.
Phones are behind NAT.

Swapping MT for TP-Link solves the problem

This is 100% configuration problem, any ideas?

Regards