This is a read-write text field, up to 32 characters, specifying digits that are prefixed onto the dialed phone number before it's sent out over the telephony interface for this peer's outgoing calls. An admin uses this when the connected telephony equipment, like a PBX, requires an access code or trunk-access digit before the real number, such as dialing an outside-line code first. For example, setting this prefix to a single access digit would make the gateway automatically dial that digit plus the actual number whenever this peer places an outgoing call.