Are you using a telephone extension lead to connect to your modem (plus an RJ11 data cable) from the Master Phone Socket?
If so, using a Master Socket to modem RJ11/45 data/voice cable of a suitable length could solve it.
I bought a 10 metre Lindy cable from click here for £7.59 including VAT, which is the cheapest I've seen for this length. The outlet is based near me so it just meant a personal call, but you can phone in an order or do it online.
On the other hand one of the offspring, also with Pipex, connects with no problems using a 20 metre telephone lead extension....:-)