(2-part post):
Ethel posted while I was composing this and I agree that it may well be your Amstrad emailer. However, I'd already written the following and, in case it's not your Amstrad device, I'll post it anyway. (It won't do any harm to try the suggestions).
It sounds like it's the opposite of broadband. i.e. it's 'dial-up'. Broadband connections can't dial numbers, whereas dial-up ones can. It seems that your PC has a dial-up modem (as well as the broadband one) and it's periodically trying to connect to a premium rate phone number.
You might have 'rogue dialler' software on your PC. See here:
http://www.bbc.co.uk/webwise/askbruce/articles /security/roguediallers_1.shtml
Use the 'check a number' facility, on the PhonepayPlus (formerly ICSTIS) website, to get details of the number dialled:
http://www.phonepayplus.org.uk/consumers/defau lt.asp
Then, if you think that your PC has been hijacked by a rogue dialler, contact PhonepayPlus to report the matter.