If you are initiating vpn client connectivity from behind the 871 to outside you need to allow through the IPsec ports udp 500, udp 4500 and protocol 50 esp. I don't know Nortel's vpn client but Im sure they follow the Ipsec security standards.

try this on your 871 router.

access-list 101 permit udp any any eq 500 log

access-list 101 permit udp any any eq 4500 log

access-list 101 permit esp any any log

apply acl-101 to your outbound interface

access-group 101 in

HTH

Jorge