Is the customer supplied doorphone a Toshiba unit or not?
If not it is usually only connected via an SLT port. If you have a DDCB then a BWDKU port will supply you with up to 3 doorphones or 2 and a release relay. I usually fit the Toshiba doorphone and get a Locksmith to provide the lock and power for the release. They usually leave you with a pair to put across your relay which can be set N/O or N/C within the DDCB.