VPN extraneous behavior. Only 6 calls allowed

[Descalante]

This is my environment:
IPO --> LAN Switch --> Fortinet 100 -->Internet -->ADSL Modem --> WRV200 Linksys Router --> IPPhone 4402

I have a VPN created between Fortinet Firewall and Linksys Router. The remote extension registers normally in the IP Office 406 V1(3.2.54). The extraneous problem is that I can make or receive only 6 calls without problems. For the seventh call I don't get dial tone although I can make the call (Call Status indicates normal connection) but the voice is not always listen in one or both ways. For incoming calls, the phone rings normally but the called side can't hear.

What suggestions can you give me to troubleshoot this issue?

Thanks in advance.

 

[ProviderReborn]

How many VCM channels do you have?
How many in/out channels does the line group have set for the IP Line.

Are you using SSA? If so take a peak if you are making out your VCM Channels.

Also what bandwidth does your VPN have and also what codec is your IP Line using?

Thanks
Lee

ACA - IP Office Implement
ACA - IP Telephony
ACS - IP Office Implement (Aug 30th)

 

[Descalante]

1. The equipment has 20 VCM channels. May be it is important to know that the system has over 70 IP local extensions.
2. I don't have a line group for the IP line. I'm not linking two IPO equipments, just one remote extension.
3. SSA is not supported because the system is version 1. Anyway I don't think the problem is caused by VCM channels exhausted. The failure was the same even with 0 traffic. Let me clarify something; I'm doing 6 consecutive calls, not concurrent.
4. VPN is through a DSL connection, so the bandwidth can be variable. At the remote site BW should be 4Mbps downgrade and 512 Kbps upgrade. Although I'm not using an IP line, all extentions are defined to work with G.729

Thanks in advance

 

[Descalante]

First of all, I made a mistake my IP phone was 4606 not 4402 as indicated above.
It appears the problem was solved using a 5610 phone. 4610 also present similar problems.
Now, what can be different in the IP behavior whit these phones?