when I ring people they tell me there is a delay on the call

I had this issue when I was using the Three network, and I still have it using Smarty which happens to be owned by Three.  I talk to a family member on their landline, and after about 20 - 30 mins a delay builds, where the family member is not hearing what I have said for between 5-10 seconds.  They also report not hearing me for many seconds some times.  I have made these calls using bluetooth earphones, without earphones, on wifi calling and without wifi calling and the same still happens.........