Asterisk on a BT Analogue Line

Introduction

Once upon a time BT succeeded in making sure that anyone trying to have equipment usable for both UK and other markets will need to go through the first few circles of hell. The official line is that this was done to enable functionality. Specifically, people refer to having caller ID prior to the phone ringing. However, the reality probably has to do with making sure that cheap imports from the far east do not work properly.

This does not really matter for phones. There is plenty of UK-specific gear to chose from, it is a royal pain when dealing with ATAs and other VOIP gear. All of these are manufactured in international or US-centric versions which struggle to cope with the BT line impedance and signalling.

BT Line Specs

The BT line technical specifications are available from BT Sinet. The specs which are of interest to an Asterisk home user connecting to analogue line(s) supplied by BT are 351 and 352. It is worth it to double check that equipment (and specific firmware) support the correct values specified in these, especially impedance and timings. I have so far tried X100P?, SP3102 and TDP400P?. None has been trouble free. I would not expect other kit to be fully trouble free either.

BT Operational Procedures

There is an additional factor besides the specs themselves which is there just to make life more interesting for an Asterisk user. It looks like some maintenance is performed on the telephone exchange every night (around 3-4 am in my area). As a result the line voltage changes once. This is treated by TDP400P? and SP3102 as the beginning of a caller ID sequence. However the caller id never comes. If the called ID in both was coded properly they should timeout shortly afterwards and reset their caller ID signalling state machines to default settings. However, both lack this timeout support. As a result Asterisk using TDP400P? stops answering calls completely. SP3102 is still capable of answering calls, but the caller ID reported for all calls will be the callerID of the last caller prior to BT maintaining the line.

Configuration for specific devices

• X100P
• TDP400P

-- AntonIvanov? - 13 Nov 2008