Just follow these instructions to resolve any VOIP phone issues with suppliers such as Sky.
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Phones are unable to provision, or phones provision but drop calls, or any random behaviour of your phone.
If your router doesn't have a SIP ALG setting under the Advanced Tab (which come sky routers don't), you will need to do the following.
To solve this problem, you will need to turn your Router on to modem, and pass it through to a third party router where you can set the SIP ALG setting. We can provide you with this router ready configured for you at a cost of £35.