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
Jorge Rodriguez