Fallback number for extension


Sorry this is long winded, but I want to explain what I have tried so far.

I have a client who has a large (150+) branch network connected via an ADSL 2+ based WAN. They want to put VoIP handsets into each branch to reduce call costs etc. So far so good.

The issue is they need to maintain telephony connections if the ADSL goes down (which they do) without having additional POTS handsets (POTS line is fine).

My solution so far is to use the Siemens IP phones in the branch, which support both POTS and SIP, configured to use SIP as default and fall back to POTS if SIP doesn’t work. This covers the outbound, but I am stuck on the inbound.

What I need is if someone from head office (or an inbound call via DDI) calls the branch extension (i.e. 5014) and the phone is offline, it calls the POTS number (i.e. 01924 225566) instead, which will ring the Siemens phone via POTS. This makes it seamless to the staff in the branch and to callers.

I have tried to implement this using follow me (initial ring time on the 5014, then the follow up group containing the 01924 number) but follow me doesn’t detect the 5014 extension is unavailable, so tries to ring it for the initial ring time before falling back to the 01924 number. On the flip side, if the branch don’t answer the phone fast enough (during the initial ring time) it stops rining on the SIP channel and starts ringing via the 01924 number, causing unnecessary call charges if they then answer.

Anyone any idea on how to do this with FreePBX 2.8 (running over Asterisk 1.6 on PIAF)? If it needs some development work, I may be able to arrange a bounty with the customer.

There must be other organisations out there with similar requirements for branch networks.