how to implement ip phone behind (Score 1)

my company is planning to give every employee a ip phone to be used in the office. our company network is behind a NAT. we can dial using the ip phone to anyone that is directly connected to the internet. but if the receiver ip phone is also behind a NAT we can dial but no voice can be heard on both side. i find out later that this is a very common problem if the NAT for both party are running on a symmetric firewall NAT which is most common for most sme network. can anyone have any solution to this problem? i would be most grateful if this is solved. thanks a million.