Turn that setting back off.
The setting is not "auto attendant transfer", but "auto attended transfer", the default setting is off.
FIRMWARE VERSION 1.0.4.9 GXP2120/GXP2110/GXP2100/GXP1450 USER MANUAL Page 27 of 68
Set "Auto-Attended Transfer" to "Yes" under Web GUI->Advanced Settings page. And then click "Update" on the bottom of the page;
Establish one call first;
During the call, press TRAN key. A new line will be brought up and the first call will be automatically placed on hold;
Enter the number and press SEND key to establish the second call;
After the second call is established, press TRAN key again. The call will be transferred;
If users press the SPLIT softkey before the call is transferred in the step above, the second call will be resumed.
Note:
• To transfer calls across SIP domains, SIP service providers must support transfer across SIP domains.
• In auto-attended transfer, use SEND key to dial out the second call instead of using #, even when # could be used as SEND in normal phone calls.