Problem with ATA 186 and network

Situation:
I have two cisco ATA 186.
I have two subnets (VPN): with IPs 192.168.170.1-255 and 192.168.171.1-225. I don’t know is there NAT or no, but I think there aren’t, becouse you can browse network without any restrictions (as a one LAN). Both subnets have their gateway pc’s – 192.168.170.40 and 192.168.171.40 (they are connected to ISP, witch provides us VPN).
In one subnet both ATA’s works fine. I use Open H323 GateKeeper for registering them.
When ATA’s are in diferent subents, in one subnet is GateKeeper and first ATA, on the ofther subnet – second ATA. And both ATA’s registers with no problems.
When I try to dial, it is a good signal, and the second ATA rings. But when ATA answers, right that moment the busy singnal comes and the conversation is over.
What could couse that failture of calling?
Please, answer here or to email vapau@aap.lt.
Great thanks for all.
Vaidas Paulauskis.