Since there is no computer daisy chained to the phone, you should be fine with configuring the vlan on the port (where the phone is connected) as an access port. If you configure it as a voice vlan, CDP informs the phone, what the voice vlan id is. CDP is proprietary, so I dont think Siemens phone will understand that. As far DHCP scope option, you can define a Predefined option with a code of 43 and specify the type of option as String (if Vendor ID is a string). Here is a link as to how DHCP option 150 is configured for Cisco IP Phones.
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a00800942f4.shtml
Note that in the above link the option type is an IP address, as the purpose is to provide ip address of TFTP server to IP Phones via option 150.
If your Building A and B are not in same vlan, which means you have a router or layer 3 switch doing the routing between the two buildings, and if your DHCP server is in a different vlan from the phones, you will need to configure the routing device as a DHCP relay agent. "ip helper-address" command will need to be configured on the phone vlan to propagate DHCP broadcasts as unicasts to the DHCP server.
HTH
Sankar
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Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus