T21P E2 - cannot be answered on siemens 3800v7

Hello,

I just bought new T21P E2 and they are not working well. I have other telephones T21 software version (34.71.0.90) and they are working well. The problem is if i dial from an old telephone (T21) to a new telephone (T21P E2) it works ok but if I dial from new telephone (T21P E2) to the old phones it does not work. the telephone rings but when I pick up it still continues ringing on the extension that i dialed from. on the dialed extension it drops and gives me the dial tone. I am using siemens Hipath 3800 V7

any help will be greatly appreciated

Thanks in advance

kenmurimi.