Help, queue generating a conference call

I have a dynamic queue where I have 1 member, this member is on the end of an spa-3102 (calls a mobile phone), when I make a call from both sip phones (a yealink and a cisco), if I hangup the first call from the spa and don’t hangup the sip device (both are handfree) asterisk places the 2 sip devices in a conference mode. when I answer the second call on the remote device I get a 3 way conference.

What is going on here and how do I stop this…