PPPoe down and voip fail

Hello,
I have more than 60 mikrotiks in differents clients with the same problem, when the pppoe is down and up again the sip voip doesn’t connect. I see in others posts and links that the problem is because off the NAT and UDP, but other routers dont have this problems. In other sites I read that this problem is because of kernel linux that uses a router OS.

+I am trying to find a solution. I changed the sip local port in all phones, I desactivated sip service from routerOS, I tried diferents scripts, for example (my sip server use 17060 port):

:foreach i in=[/ip firewall connection find where dst-address~“:17060” protocol~“udp” and assured=no] do={
/ip firewall connection remove $i
}
OR
:foreach i in=[/ip firewall connection find dst-address~“:17060” protocol~“udp”] do={
/ip firewall connection remove $i
/log waring message=“kill connection”
}

This sometimes works and sometimes it doesn’t. Sometimes it fixes a few phones but sometimes it doesn’t
I need o to find a definitive solution to fix it but I can’t change the voip to TCP for fix it…

I dont have this problem with others router that are worth $ 40 but dont have all the functions that I need…

Links with more info about my problem:
http://jkroon.blogs.uls.co.za/it/voip/connection-tracking-problems
http://forum.mikrotik.com/t/mikrotik-router-sip-connection-blocked/31641/1
http://forum.mikrotik.com/t/obsolete-connection-table-entries/115586/1

I would appreciate any help, Thanks.

PD: im using masquerade for pppoe

Because that problem I stopped to buy Mikrotik routers, I start to buy Edgerouter and dont have this problem, I like better Mikrotik but with this problem I cant mount it.