Call parking/transfer fails at random

[Jaap]

1. Running 7.70.13.4 and somehow upgrade to 7.70.0.140 fails.
2. Attended transfering fails, so A talks to B. A presses tran+C-number#. A talks to C and then tran presses tran to connect B to C. When the call is placed in the waiting queue correctly waiting music will sound (and transfering goes as it should). When there's no music, transfering fails.
3. When only using hold the same symtoms occur. Sometimes the holding music and sometimes nothing. We can always get back to the waiting call, so it parks the call.
4. We're using a hosted PBX system. It's hosted at sip.sipnl.nl.

Hope this give a little more information.