The telemarketers use a special chip in your phone that operates separately - it's the one that senses when you're about to eat tea for them to ring !!
One thing that might be causing it if the fabric seems clean, is the adhesive that was used originally. I suspect it was made of something very strange as when I've stripped back cases for restoration the view underneath the fabric is usually pretty gross :
It may be worth removing the fabric gently, cleaning it with a cotton bud and mild detergent and removing the funk you find behind the fabric - then using fresh adhesive once it's clean. Halfords sell a good adhesive to use - it's an aerosol of adhesive that is used for sticking fabric to parcel shelves in cars. It won't seep through the fabric and dries almost instantly as a contact adhesive on both surfaces and then just press together.
All the best,
Jay