Phone interference from router

Not exactly sure if i'm in the correct place for this but i'll give it a go. I recently purchased 3 cordless phones for my house. When I set them up I tested one out and when ringing somebody I could hear a fuzzy echo from the ringtone and then from the persons voice on the other end. My voice sounds fine to them and I have tested ringing the new house phones myself from my mobile and can't hear any problem. It seems the issues is with the cordless phones.

The main phone base was connected to a filter with the other line going to my router. I decided to remove that and just place the main phone line into the wall. It seems the interference is greatly reduced so I can't see its a technical fault with the phones.

My problem is what do I do now? I need the router connection and there is no other phone point to connect it to. I must use the filter to get both the phones and the router. However there must be something to stop the interference on the phone which I am not aware of yet. I hope somebody can help me out. Thanks.