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The old AT&T phones (Yealink) require additional configuration to fully work with the Grandstream UCM.
Requirements:
SIP Account Setup
The SIP setup for the extension is pretty standard. Extension number and password from the UCM. Endpoint 10.141.0.2.
Voicemail Config
Yealink phones don't know how to contact Grandstream voicemail by default. In order to receive voicemail notifications, you have to go to Account → Advanced → Subscribe for MWI. This is set to on, and voicemail is set to *97. I typically add a soft key for Voicemail under Dsskey.
Directory / LDAP Config
The directory config is the most complicated.
Enable LDAP: Enabled
LDAP Label: PPS
LDAP Name Filter: `(
LDAP Number Filter: `(
LDAP TLS Mode: LDAP
Server Address: 10.141.0.2
Port: 389
Base: dc=pbx,dc=com
Username: admin
Password: admin
Max Hits: Default
LDAP Name Attributes: cn sn displayName
LDAP Number Attributes: telephoneNumber
LDAP Display Name: %cn
Protocol: Version 3
LDAP Lookup for Incoming Call: Enabled
LDAP Lookup for Callout: Enabled
LDAP Sorting Results: Default
T48S Phone
The T48S model allows a background to be uploaded. See link: 