It could be either a fault your end or a fault in the exchange so you will need to do some checks.
Have you another phone you could try? Maybe a corded one?
It could also be someother equipment in your house causing the problem, Broadband filters are known to cause an issue, as are some varients of the home hub.
Maybe worth plugging a phone into the test point of your main phone socket with nothing else connected.
If after doing all this there is still a problem give BT a call (I assume they provide your service?)
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