[Freeswitch-users] Phones become unreachable after some time

I use FS 1.0.3 with Grandstream HT386 and X-Lite soft phones, FS binds to Mysql DB for SIP registrations, presence etc. I noticed that after some time probably >30 min. phones which have been registered but without making calls become unreachable. Meaning that any call to such extension gets forwarded to VM as if it was offline, until I reload such phone. I did try to make the phones to register every 5 min. but it does not help. I also see valid registration information in sip_registrations table. X-Lite has r-port and keep alive settings on. Would appreciate any hints on what can be the issue here.