[Asterisk-Users] problem connecting multiple boxes via IAX2

[MobilPete]

Senerio
multiple * boxes connecting to a central * box with T1 card via IAX2.
1>box 1 abd 2 work fine all the time
box 3 - after approx 10-15 minutes with no calls - central box with T1 card 
fails to deliver incoming calls to box 3.
Connectivity is good, * exten-2-exten good

in order to allow incoming calls again, we only need to make 1 outbound call 
from box 3. Then everything works well again.
Can anyone shine some light on this problem?