W52P - 1 of 3 handsets replies as busy, but it is not busy

[mkeuter]

Hi,

I have an issue with a W52P with 3 handsets (FW: 25.73.0.40 + 26.73.0.11).
When I make a groupcall to the W52P so that 2 or all 3 accounts are ringing at the same time, in 60% of those calls the first SIP account of the base (102) replies with SIP response 486 "Busy here", although the account is not busy.

If I call each account separately it works fine all the time.
I switched the handsets (on different accounts), resetted the handset, made a factory reset on the W52P, but still the 1. account gives me busy very often.

Code:

-- Executing [333@groups:1] NoOp("SIP/108_yeal42g-0000016b", "Groupcall Ext. 333") in new stack
    -- Executing [333@groups:2] Dial("SIP/108_yeal42g-0000016b", "SIP/102_yeal52p&SIP/120_yeal52p,,t") in new stack
  == Using SIP RTP TOS bits 184
  == Using SIP RTP CoS mark 5
  == Using SIP RTP TOS bits 184
  == Using SIP RTP CoS mark 5
   -- Called SIP/102_yeal52p
    -- Called SIP/120_yeal52p
    -- SIP/120_yeal52p-0000016d connected line has changed. Saving it until answer for SIP/108_yeal42g-0000016b
    -- SIP/102_yeal52p-0000016c connected line has changed. Saving it until answer for SIP/108_yeal42g-0000016b
    -- SIP/102_yeal52p-0000016c is ringing
    -- SIP/120_yeal52p-0000016d is ringing
  == Spawn extension (groups, 333, 2) exited non-zero on 'SIP/108_yeal42g-0000016b'
    -- Executing [333@internal-phones:1] Goto("SIP/108_yeal42g-0000016e", "groups,333,1") in new stack
    -- Goto (groups,333,1)
    -- Executing [333@groups:1] NoOp("SIP/108_yeal42g-0000016e", "Groupcall Ext. 333") in new stack
    -- Executing [333@groups:2] Dial("SIP/108_yeal42g-0000016e", "SIP/102_yeal52p&SIP/120_yeal52p,,t") in new stack
  == Using SIP RTP TOS bits 184
  == Using SIP RTP CoS mark 5
  == Using SIP RTP TOS bits 184
  == Using SIP RTP CoS mark 5
    -- Called SIP/102_yeal52p
    -- Called SIP/120_yeal52p
    -- SIP/120_yeal52p-00000170 connected line has changed. Saving it until answer for SIP/108_yeal42g-0000016e
    -- SIP/102_yeal52p-0000016f connected line has changed. Saving it until answer for SIP/108_yeal42g-0000016e
    -- SIP/120_yeal52p-00000170 is ringing
    -- Got SIP response 486 "Busy Here" back from 192.168.222.159:5062
    -- SIP/102_yeal52p-0000016f is busy