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As noted above, SIP ALG is just one piece of the larger whole that is your home network. If disabling SIP ALG does not help resolve problems with your NocTel handset set up at home, a strong next step to resolution is to contact your Internet Service Provider and inquire if they allow the common ports used for VoIP or have possibly had any changes to allowing VoIP traffic. (List of ports used by NocTel - Polycom-based)

Small and Medium Business Combo Modems

Small and medium businesses may exhibit compatibility issues with NocTel handsets based on the Internet Service Provider, volume of handsets, and volume of call activity relative to the connection service. Such hardware are often referred to as "combo modems" due to the fact they are often equipped with a modem coax connection (to feed internet service into), LAN ports for devices/switches, integrated wireless router, and/or RJ-45 port for landline phone service. Due to the consolidation of many networking aspects into a single device - typically with the intent of using a single device rather than multiple for a specific service when selling bundled services (voice, internet, and cable) - the standard operation of combo modems may either not be configured properly or in some cases not be configurable to be compatible with NocTel service. 

However, there are some options for workarounds:

1. Connect a router (see list above) to a LAN port of your combo modem and place this router into Bridge Mode. No additional configuration of this router should be necessary.
2. Several internet service providers allow residential customers to purchase and use their own modem/combo modem with their service rather than an ISP issued modem. If your ISP allows, purchase a combo modem that is known to be compatible with SIP. This option also allows you to save a bit of money each billing cycle as you are using your own equipment and cannot be assessed a modem rental fee by your ISP. However, before purchasing a modem/combo modem, be sure to verify it's compatible with both your ISP and the service option you are subscribing to. Several modems, such as the Netgear C7000 often have notes stating "DOCSIS 3.0" or "compatible with Comcast Xfinity" - be sure to gather relevant information on a potential replacement modem to ensure not only the best likelihood of SIP compatibility but also that the speeds your connection allows can be utilized.

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