Direct SIP URI functionality is not available in the Voip Client at the moment.
In Version 567, you can insert letters in the voip client. But the reason for this is not for direct SIP URI dialling.
We enabled the ability to type letters in the dial pad area for Billing function purposes only for now.. however we plan to include calls to SIP URI soon.
Billing dial code - FYI
You would like to make a call to 102. So you dial 102**Accounts. PBX will make a call to 102 and in the logs will generate the INVITE in this format
CM500002]: Info on incoming INVITE:
INVITE sip:102**accounts@10.172.0.2:5060 SIP/2.0
Therefore you can trace calls made with that code for billing, reporting purposes, crm etc etc.