FritzBox7270 + ISDN plant: Telephone how internally?

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Since short I operate a Fritz box 7270 with a “attached” AGFEO plant at the S0 bus.
The 2 DECT of telephones run now over the FB. I cannot telephone however to the similar participants of the Agfeoanlage internally.
Does someone know a possibility/a key sequence of DECT1/FON1 e.g. to the branch 20 to select?
How does that decrease/go back? (Thus branch 20 to FON1 or DECT1)?
Thank you
Greetings

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Problem as follows solved: (after tap from the forum via report of private)
S0 plant got internal number 50 assigned.
in Agfeo plant: MSN 51-59 put on. These can be called by DECT telephones at Fritz box with ** 51-59. (further MSN e.g. MSN1 12345)
However unfortunately is something unkomfortabel, since with the calling *5*MSN1* appears. So I cannot differentiate between the individual internal connections. Probably that runs also not “internally” separates over the MSN1, i.e. probably liable to pay the costs (however a Flatrate for Festnetztelefonie has).
Does someone have still another better idea?