VoIP-to-PSTN termination providers and SIP vendors will be watching their inboxes for a lawyer's letter from BT, which has kicked off a taxing licensing program levying a fee on the industry, based on a list of 99 patents.

As noted in Australian telco newsletter Communications Day, the move seems to have caught the VoIP industry by surprise, with SIP Forum chair Richard Shockley saying the move has shocked the industry and is already frightening smaller players.

The British incumbent is offering to allow third parties to use the Session Initiation Protocol under a license agreement published here.

In the license, BT is requesting either $US50,000 or a combination of 0.3 percent of future revenue from affected products, plus 0.3 percent of the last six months' sales for products as “past damages”. It's kindly offering a discount for customers that pay up within six weeks of receiving a BT letter of demand, and there's a premium to $US60,000 and 0.36 percent of revenue for those who hold out.

Targets also have a choice of an annual license fee of $US12,000 or 0.3 percent of the previous 12 months' sales for the product (with similar discounts and penalties offered for moving quickly or slowly as the case may be).

The license looks to The Register to be designed to encourage quick compliance among small companies who probably wouldn't win a court case anyway, since the giants of the networking business are unlikely to comply anytime soon.

It's probably no coincidence that BT and Google have been firing exploding lawyers over each others' parapets since 2011. BT kicked off the tit-for-tat-spat with a filing targeting Android in a 2011 filing in Delaware.

Google's February 2013 response included asserting a patent covering a VoIP gateway between a phone and the Internet.

SIP provides a standard syntax for opening sessions across VoIP gateways. It's vital both to VoIP service operators running PSTN termination, and to the vendors that supply the industry. ®