We have had reports of the audio (both directions) breaking up on SIP2SIM calls. We have contacted the mobile carrier to investigate this as the break up is happening between our SIP servers and the mobile carrier.
The problem with audio is intermittent and proving hard to track down. WE do have the support of the mobile carrier and we are trying to reproduce the problem.
We've still not been able to reproduce audio break up on the test calls we're making, they have all been fine. We still have this issue open with the mobile carrier and will continue to investigate this.
Since the reported problems we've not been able to reproduce the audio break up, customers have also reported that the problem has gone away. We'll re-open a new incident and will investigate with our carrier if/when this problem resurfaces.