My voip calls to 101 always cut off 6m 36s to 6m 37s

[CambridgeWorks]

We keep our old tel number as some friends or relatives may only phone once in a blue moon and occasionally someone from several years ago rings unxpectedly and landline is only number they have and we don't have details in an address / phone book.
We also have 3 dect phones scattered throughout the house.
Rob