2nd Channel specification

Hey,
We are using Asterisk for making calls. We are doing this using one PRI (T1) line. As the number of users are increasing we need to connect calls using second PRI (T1) line, if the first PRI (T1) line channels gets exhausted. How can I do this? Is there a way to do this without disturbing the context in the dialplan?
In /etc/asterisk/zapata.conf, I have a group (group = 2) and I have specified Channels which I need to connect using the first PRI (T1) line. Another group (group 3) has been defined mentioning the bchannel of second PRI(T1) line.
In /etc/asterisk/extensions.conf, I have specified the dialplan with Zap/g2 as trunk and its active and working. But when I specified Zap/g3, it doesn’t work.
Please help me with this issue. Thank you in advance.

Regards,

Kurian Mathew Thayil.