Voip and VPN, voice interrupt - what can i do?

[Anuelicon]

Hi to all, i'm anuelicon, a new user. And i begin with a probelm that i still have at my company.
This is the scenario:
Work place with HG1500 and CISCO router in VPN with a 3com router at home. IPPhone connect in both side, at work and at home.
The VPN works correctly, sharing, file tranfert etc...
The IP phone at home can ping the IP phone at company and viceversa, call and response...
And now..the problem...The converation is ok for 17 seconds, then, 3 seconds of silence, it seems like if the microphone is automatically disable, then other 17 seconds of conversation ...etc etc.

Why this precision? The call never goes down...after 3 sec the person of the other side can hear you...
What tests can i do for resolve this problem, and what is the couse of this disservice.
May be the packet size ?!?! Or VPN ?!?!
Many tnx to all, and sorry for my english (basic level ).