They will have got your number from an aggregator like confused.com or the like - you've gone looking for quotes and they've paid someone for your details.
If you're TPS registered they can only contact you if you explicitly consent to the contact - watch out for the small print on aggregator sites...
Complain via Ofcom; here's the details from Ofcom's website
How can consumers complain if they receive a silent or abandoned call?
If you have not done so already, you should contact your network or service provider. Your provider might be able to identify the organisation making the silent or abandoned calls and provide you with advice or assistance to prevent such calls in the future.
If you continue to receive silent or abandoned calls and your network or service provider is unable to assist, please contact Ofcom’s Contact Centre to register a complaint:
Web: www.ofcom.org.uk
Phone: 0300 123 3333
Fax: 0300 123 0811
Post:
Ofcom Contact Centre
Riverside House
2a Southwark Bridge Road
London
SE1 9HA
Ofcom's Contact Centre logs all complaints under a case reference number. Ofcom will update our database of complaints accordingly. This information will help Ofcom to target and take action, where appropriate, against organisations that are making these types of calls