the following is my current entry in extensions_additional.conf (excluding the dots at the beginning and end:

...
[ext-did]
include => ext-did-custom
exten => fax,1,Goto(ext-fax,in_fax,1)
exten => s,1,Set(FROM_DID=s)
exten => s,n,Gosub(app-blacklist-check,s,1)
exten => s,n,Goto(timeconditions,8,1)
exten => _X.,1,Noop(Catch-All DID Match - Found ${EXTEN} - You probably want a DID for this.)
exten => _X.,n,Goto(ext-did,s,1)

; end of [ext-did]
...

by inserting "exten => s,n,SIPDtmfMode(inband)" without the quotes after the line "exten => s,1,Set(FROM_DID=s)" it works the way i want it to, but will be erased the next time a change in AMP is made.
therefore i'd like to use extensions_custom.conf, but i have problems getting this to work.

here is my entry at the end of extensions_custom.conf (i've also tried replacing '2' with 'n'):

...
[ext-did-custom]

exten => s,2,SIPDtmfMode(inband)

; end of [ext-did-custom]