To configure directly on the device web page, the syntax is:
pp(ob290123456)
Substitute 290123456 w/ your obi number. Note the use of the "ob" prefix.
You can replace pp with other trunk prefixes: sp1/2,li, as needed.
The "ob" prefix is only needed for the pp trunk.
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**9 or similar prefixes are used in dialing only (PHONE Port and AA Service).
You can pickup the phone, dial *62, followed by **9 & the the obi number.
Note that default *62 behavior is to apply the same call forward to all trunks.
Similarly dial *63 to cancel it.
If you use *62 **9 ..., method, you will also notice the same
format is used by the device to store the call forward configuration.
In other words, the call-route for forwarding call is determined at the time
the cal forward is configured; not when the call is forwarded.