why do i get "startup-config file open failed (not enough space)

[mid230]

Hi all, we have 3745 cisco with 12.3(6) ios.... today i tried to save the running config to the start-up config i get an error "startup-config file open failed (not enough space)" .. the router is used for voip and it is running calls fine... and when i do 'sh startup-config 'command this is my output:
Using 48468 out of 155640 bytes, uncompressed size = 139270 bytes
%Error opening nvram:/startup-config (Not enough space)

I am not sure what is going on and don't know how to resolve it.....

any idea will be very helpful..

thank you