If you place a call using using BLF or the Directory and then want to call another user or conference or transfer that call, the phone, by default, is in URI mode and prompts the user to enter a SIP Address. The simple solution is to hit the "#" key which puts the phone back in number dialing mode. You can then enter the phone number or extension to transfer or conference that call.
If you use the Polycom phone's BLF buttons or Directory to place a call, from that point on, for the duration of that call, the phone is in SIP URI mode meaning that it's expecting to call via SIP address. This is due to the BLF keys and Directory dialing using the SIP address of the user as the way to place the call. The simple solution is to hit the "#" key which puts the phone back in number dialing mode.
Typically this is not an issue. You place the call using the BLF key as a speed dial or find the user via the Directory and after the conversation you then hang up the call. The phone then returns to number dialing mode for the next call.
**You *can* completely remove URI dialing from the phone which removes this issue, however it also disables the ability to use the Directory on the phone to place calls. OnSIP Customer Success team has access to a file you can load on the phone to completely disable URI dialing, but be aware of the side-effect.