logic 
if no active call for context, make call
else if active call on context is same as number dialed hang it up
else hangup any other active calls on context and call the number dialed

[100]
exten => 456,1,Set(CURRENTNUMBER=9999)
exten => 456,n,Set(CURRENTNUMBER=${DB(100/NUMBER)})
exten => 456,n,GotoIf($[${CURRENTNUMBER} = 9999]?MAKECALL:CHECKNUMBER) 
exten => 456,n(CHECKNUMBER),GotoIf($[${CURRENTNUMBER}=456]?HANGUP:MAKECALL)
exten => 456,n(MAKECALL),HangUp()
exten => 456,n,Set(DB(100/NUMBER)=456)
exten => 456,n,DIAL(SIP/456,20,gWW)
exten => 456,n(HANGUP),HangUP();
exten => 456,n,DBDelete(100/NUMBER)

exten => 123,1,Set(CURRENTNUMBER=9999)
exten => 123,n,Set(CURRENTNUMBER=${DB(100/NUMBER)})
exten => 123,n,GotoIf($[${CURRENTNUMBER} = 9999]?MAKECALL:CHECKNUMBER) 
exten => 123,n(CHECKNUMBER),GotoIf($[${CURRENTNUMBER}=123]?HANGUP:MAKECALL)
exten => 123,n(MAKECALL),HangUp()
exten => 123,n,Set(DB(100/NUMBER)=123)
exten => 123,n,DIAL(SIP/123,20,gWW)
exten => 123,n(HANGUP),HangUP();
exten => 123,n,DBDelete(100/NUMBER)

[200]
exten => 456,1,Set(CURRENTNUMBER=9999)
exten => 456,n,Set(CURRENTNUMBER=${DB(200/NUMBER)})
exten => 456,n,GotoIf($[${CURRENTNUMBER} = 9999]?MAKECALL:CHECKNUMBER) 
exten => 456,n(CHECKNUMBER),GotoIf($[${CURRENTNUMBER}=456]?HANGUP:MAKECALL)
exten => 456,n(MAKECALL),HangUp()
exten => 456,n,Set(DB(200/NUMBER)=456)
exten => 456,n,DIAL(SIP/456,20,gWW)
exten => 456,n(HANGUP),HangUP();
exten => 456,n,DBDelete(200/NUMBER)

exten => 123,1,Set(CURRENTNUMBER=9999)
exten => 123,n,Set(CURRENTNUMBER=${DB(200/NUMBER)})
exten => 123,n,GotoIf($[${CURRENTNUMBER} = 9999]?MAKECALL:CHECKNUMBER) 
exten => 123,n(CHECKNUMBER),GotoIf($[${CURRENTNUMBER}=123]?HANGUP:MAKECALL)
exten => 123,n(MAKECALL),HangUp()
exten => 123,n,Set(DB(200/NUMBER)=123)
exten => 123,n,DIAL(SIP/123,20,gWW)
exten => 123,n(HANGUP),HangUP();
exten => 123,n,DBDelete(200/NUMBER)