A common problem which I have seen during point to point circuit testing is an Encapsulation problem. If you work in an organization where you don’t have a control on the other side then you know what I am talking about.
I have seen that if you are backbone guy and you are setting your router interface to encapsulation ppp. You will somehow tend to assume that engineer on the other side is also using ppp encapsulation. In most of the cases this assumption always leads to wastage of time in circuit testing.
By default on all cisco interfaces, encapsulation is HDLC ( cisco proprietary) . During testing of new point to point links, always try to verify encapsulation at both ends, if on one side you have hdlc and on other side you have ppp, then your link will never come up.
If you don’t have a control on the other side of the link then either try to configure encapsulation ppp at your side or if you have already configured ppp on your side, try removing it.
Router1(config)# interface serial 0
Router1(config)#interface serial 0
Router1(config-if) no encapsulation ppp
Changing encapsulation always work.