Caution! A non-migration of a broadband + voice service may invoke termination charges. To avoid this you might be required to give 30 days notice. Also if you prematurely stop the direct debit mandate, you might find yourself paying an admin charge for the alternative method of payment to the loosing ISP.
Just re-reading your post... OK I think you're making the distinction between a traditional migration of broadband + voice landline to another provider, and the porting-out of a number to a VOIP provider. I see. I guess it's a case of reviewing the losing/donor providers T&Cs then.
I am actually in the process of porting out a number from OneCom to an VoIP provider. It just so happens that the number is "silver", if not "gold", and so I didn't want anything to go wrong. The contract for Broadband + Voice expires mid October and intended to let the service die on the vine, porting out the number in mid September hence decided to read in depth their T&Cs. There they state that at least 30 days notice is required to terminate the service otherwise termination charges apply; what is not particularly clear is whether it is a calendar month or not. No worries the notice has gone in now.
Aware that others have said that payments continued to be taken after a contracted had ended, considered scrapping the DD and paying by BACS as the monthly bill came in, only to discover there was a hefty charge for non DD payments, Once the final payment has gone through, then the DD will be withdrawn.
In the meantime a new service has gone in with the existing services not used and particularly with the voice service having an attached phone. Will the porting go through without a hitch? Fingers crossed.