Callforwarding Problem

dear community,

first of all: my Trunk is up and running.
have 2 extensions 10 and 20.
i can made outbound calls, and inboud calls, without problems.

on both extensions i have a snom phone.

so now i can: call out from 10 and 20 to PSTN.
call from PSTN to 10 and 20.

and now here comes my problem:
if i forward (ext 20) a call to external PSTN, if i call from external, all is forwarding fine (i get the correct extension)
but if i call from my ext 10 to the forwarded ext2 from internal, the call breaks with 480.

short: internal forwarding to external number not working. but if i call from external the ext 20, all working fine.

so now i call here from ext 20, and not from +PUBLICNUMBER20.
i have done sip debugging, the diversion header = sip:[email protected] but my provider needs +XXXXXXXXX20 Format.

What is my problem with them?

please help, thank you very much