first just stick will
the one way
in the DN setting of the ip phone lets say you have phone with number of 1234
under the external number mas in 1234 DN setting put it like 8522996XXXX ( assuming that ( 8522996xxxx) the number that the teleco expect from you
under the route pattern that route the calls to the PSTN check the Checkbox for Calling Party's External Phone Number Mask ( as you did )
do not do other trasformations to the calling number remove the setting you did in point 3
after that, if dosen 't work
debug the gateway
by enabling debug isdn q931 then make a call and see if you sending the calling number to he telco if Yes thats mean the telco is overriding the calling number
good luck
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