X21 & ADSL configuration on 2600 router

2600 router with 12.3(8)T10 IOS WIC1T , 1ADSL and ethernet 4E cards installed.

At the moment I have a wireless network sharing a 2meg ADSL. I now also have X21 2meg available which I wish to utilise for voip services but also load balance with the ADSL. The above will need to sit behind NAT and have DHCP. I need help with this configuration please.

Thanks,

KJ