Its probably supplied to suit all phone systems with a suitable adapter.
Normally one is supplied in the box with it and is actually shown on their page, so perhaps you have discarded it by mistake?
As I have mentioned, an unused microfilter would do the same job.
Assuming that you bought if from a retailer, it should have the correct connector supplied. I'd suggest querying that with them. Otherwise just look for an RJ11 to BT adapter or as previously suggested, an old microfilter will do the job, albeit a bit clumsily.
If you have old equipment laying around with an RJ11/BT cable you could try that in place of the new cable. It will either work or not, but won't do any harm.
