EPM: Improve Transfer Handling on Yealink and Grandstream phones

The transfer key on Grandstream phones default to blind transfer.
I believe the best default for EPM to use on Grandstream phones is Auto Attended Transfer (P1376=1) where the user can choose between blind and attended transfer every time and the user can do this using the transfer button and without having to remember key sequences to dial.

From the documentation:
• Blind Transfer

• During the first active call, press TRANSFER and dial the number to transfer to;
• Press SEND key or # to complete transfer of active call.
  • Attended Transfer.
• During the first active call, press LINE key. The first call will be put on hold;
• Enter the number for the second call in the new line and establish the call;
• Press TRANSFER;
• Press the other LINE key which is on hold to transfer the call.
  • Auto-Attended Transfer.
• Establish one call first;
• During the call, press TRANSFER. A new line will be brought up and the first call will be automatically placed on hold;
• Dial the number and press SEND or # to make a second call. (Once the number is entered, a "Transfer" soft key will show. If "Transfer" soft key is pressed instead of SEND or #, a blind transfer will be performed);
• Press TRANSFER again. The call will be transferred.
• For Auto-Attended Transfer, after dialing out the number for the second call, a "Split" soft key will show. If the second call is not established yet (ringing), pressing "Split" will hang up the second call. If the second call is established (answered), pressing "Split" will resume the second call and keep the first call on hold.

Option is:
P1376=1 (to enable auto attended transfer)

For Yealink there is a similar issue. I believe the best option is
transfer.dsskey_deal_type = 1

From the documentation (about the transfer.dsskey_deal_type property):
It configures the DSS key behavior during an active call when user presses the DSS key and the DSS key is configured as a speed dial, transfer or BLF/BLF list key.
0-New Call
1-Attended Transfer <-- I think this should be the default
2-Blind Transfer
The default value is 2.