There appears to be some incompatibility between DrayTek and Telstra’s ISDN equipment or service which results in Call Bumping not operating fast enough for a phone call to be established. One of the “B” channels is dropped, but by this time the caller hears engaged or unavailable tone. For outgoing calls you can replace the telephone receiver then pick up immediately to get the outside line. If the incoming caller rings back straight away the call will go through – but most callers won’t know to do this.

DrayTek have significant expertise with ISDN, particularly in the UK and European markets. They have advised that this is a problem peculiar to Australia, that the problem is at the Telstra end, and that they cannot fix it.

We consider this a serious issue and have decided to sell ISDN models only after ensuring that the customer understands the Call Bumping issue, and accepts that our sale is with the condition
that Call Bumping does not work fully in Australia.

The effects of Call Bumping will be minimised or avoided if the customer:

  1. uses the ISDN for data only (i.e. ISDN is not used for voice calls), or
  2. selects Link Type of “Dialup 64Kbps” then only one B channel is used, so the second B channel is always available for voice, or
  3. has only light internet usage (BoD will only occasionally use the second B channel), and infrequent incoming calls, then the probability of both occurring at the same time is low. The customer may be prepared to accept that occasional calls will not get through.