VOIP Handset woes

SAbboushi
SAbboushi used Ask the Experts™
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Have a Comcast business account with 50Mbps/10Mbps service using CISCO DPC3939B Gateway.  Not using Comcast's VOIP XFINITY service.

VOIP Handset and PC are hardwired to amped|wireless AC750 Range Extender TAP-EX2 configured to use dual band.

VOIP handset drops calls.  On one instance, call was dropped and couldn't get dialtone without rebooting the AC750.  Before reboot, the 5GHz signal was at 0% (per AC750 panel display) whereas the 2.4GHz signal was at 97%.  I'm not sure of the relevance since the AC750 is configured to route hardwired connections through the 2.4GHz connection.

Wanting to know best practices for configuring the AC750 and CISCO DPC3939B for VOIP.  Also, whether AC750 is a satisfactory choice: I need a wireless connection between (a) the pc and VOIP handset  and (b) the Gateway.  

I don't see QoS settings on the AC750 nor in user manual for the Gateway.  I also saw a post here saying that known problems with CISCO DPC3939B Gateway include:
SIP ALG is enabled by default and cannot be disabled, causing intermittent phone call and feature failures.

Any thoughts on that?

Is resolving the dropped VOIP calls likely going to chew up a lot of time diagnosing and might it be better to subscribe to Comcast's VOIP/XFinity -- although it seems I would still have the need for a wireless hop between the handset and the Gateway's Line 1/2 ports...?

Appreciate any help -- thanks.

edit: Seems Comcast's VOIP/Xfinity is for use with analog handsets...
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Distinguished Expert 2017

Commented:
Do you have qos set on the ac750 to prioritize VoIP traffic, did you disable sip related features on the ac750 as it often interferes with VoIP remote setups, check with them....
It sounds as though your setup is double nat, the Cisco natting to ac750 natting to phone.

Look at replacing the Cisco or see if it can be configured in a bridge mode meaning it will be the end point on the coax feed, but all handling will be passed to the ac750 ....

Author

Commented:
Do you have qos set on the ac750 to prioritize VoIP traffic, did you disable sip related features on the ac750
I don't see QoS or SIP settings on the AC750

see if [the Cisco Gateway] can be configured in a bridge mode meaning it will be the end point on the coax feed, but all handling will be passed to the ac750

Can you 'dumb that down' for me?

Author

Commented:
I found this link re: bridge mode.  How might this help?
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Distinguished Expert 2017

Commented:
The cisco gas sip alg enabled by default and can not be changed, Cisco in bridging mode if available will /should remove the sip alg part is it will not be handling routing/nating.

I believe on the ac750 dip alg is disabled by disabling sip passthrough on the same page where you have enable VPN, IPSec, passthrough.

Check with xfinity on whether you can set the device if rented to bridging mode.
Distinguished Expert 2017

Commented:
In bridging mode traffic pass over the Cisco device to the ac750 without any additional interaction after the signal from the coaxial feed is converted to Ethernet traveling over the bridge terminating on the ac750 whose WAN port will now reflect the public IP assign/subscribed by Comcast.
Versus currently your ax750 wan port has a 10.x.x.x or another private IP range.

Author

Commented:
thanks-

I believe on the ac750 dip alg is disabled by disabling sip passthrough on the same page where you have enable VPN, IPSec, passthrough.
I don't believe this device has configs for vpn, IPSec, QoS, voip... (User Giude)

If I put the Gateway in bridge mode, will this disable the dual-band wireless component of the Gateway too?
Distinguished Expert 2017

Commented:
You mentioned it, but I seemed to have overlooked such that I took as though you have two routers.  You are using ac750 range extender which does not have the routing option/functionality that is needed to place Cisco into bridging mode. Check with Comcast to see which routers they have to offer in your area and see if one of the others has the option to disable sip alg.

Sip alg has an effect on VoIP calls.

Author

Commented:
Thanks arnold.  Is there a way to confirm that the Comcast Gateway's Sip alg is causing the problems, e.g. maybe the range extender's logs or some other tools?
Distinguished Expert 2017
Commented:
Any VoIP cloud provider includes in their instructions the need to disable sip alg.
The issue is that sip/VoIP traffic is time sensitive and needs to get to its destination with no interference from any device along the way.

You could use wireshark, tcpdump, ms network monitor to capture packets making sure your time is synchronized to external source and then looking at the provider's call logs as well as any phone logs/debug to determine the cause for the dropped call. I.e.

Author

Commented:
Thanks arnold

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