The company that looks after my clients internal phone system has a problem with logging in to the PABX using their data modem.

Connection looks like this

ISDN PRA from telco -> Asterisk -> SIP Trunk to my clients Asterisk ->
my clients Asterisk -> E1 port to his old PABX

I am planning to use transfer capabilities on DAHDI channel to my clients PABX. Should I also set it on sip trunk and channel from the telco in order for it to work?

Best regards

Andrew