SIP device connected with JustCall number should be allowed to transfer/merge calls

There should be a feature to initiate call merge directly from a SIP device, as this functionality is not currently available in JustCall. A SIP device connected with a JustCall number should also be able to transfer calls directly to any other number without relying on the JustCall dialer.

Adding these capabilities would provide more flexibility for users who primarily use SIP devices and ensure consistency in call handling across platforms.