The picture of your last post was of a DKT 2010. Some slight differences. They both work on the same systems. The codes change depending on the tyoe of system you are using.

The manual you show is for a new Strata CIX or CTX models. You likely have an older Strata DK phone system. Not a secret, just the wrong manual.

As mentions #604 is the correct code, but the redial key saves changes.

The entire squence is #604 850 Speeddial {2 digit seconds} redial.

Now you may wonder where the speed dial key is located. Try the very top button. Also try substituting * for speeddial. If that doesn't work, just press redial after 850.