plug RJ11 TO 837 cisco router

Try plugging the rj11 into the rj45 jack on router and see if it locks.

Not sure it there is a difference in the pin geometry i.e the spacing between the pins.

There are adapter for this purpose so just google for "RJ11 to RJ45 adapter" - you should get lots of hits

837 does not have an RJ45 for the DSL. Unless of course my memory is worse than I remember.

It has an RJ11.

I suppose yours may have been made incorrrectly or changed later, however none of the 837's that I have seen have ever had an RJ45 ADSL connector.

here is an illustration of the rear of an 837 showing the ADSL RJ11 port

Notice that the ADSL port is on the right-hand side when viewed from the rear

The LAN interfaces (RJ45) are on the left-hand side when viewed from the rear

Is this where you were trying to connect the ADSL line to ?